Could you please tell me the reason behind it?

Yes certainly - it goes like this -
Petrol is sold by volume - i.e Liter .
Density of petrol varies with temperature - the density of petrol is higher at lower temperatures and vice versa .. which is like most other usual substances which expands when heated .
Petrol pumps/bunks keep their petrol in underground tanks . Now these tank's get heated with the soil all day in the sun and cools down all night . They attain their max temperature at the end of the day just before sunset and minimum temperature just before sunrise ( dawn ) . So the petrol they contain is at it's highest density just before dawn too . Check ?
So when you fill near about dawn the petrol is at it's highest density ( for the day ) and you are getting more real weight of petrol for your liter (volume). Actually that dawn liter would expand in your bike/car tank to more than a liter , as the temperature rises during the day .... Hence your bike/car/whatever would run more for the same one liter of petrol you bought . Check ?
Simple , nahi ?
This would not have worked if they were selling petrol by weight . i.e kilogram .

Add to this the larger vaporisation loss during the fill ; say at mid-day , compared to when the petrol is cooler at dawn .

^^..Good information...Thanks

Can someone elaborate on this point?
How does mere gear switching with holding clutch (& close throttle) eat fuel?

I always thought fuel consumed is directly proportional to throttle opened(ignoring idle rpm overhead).

the density variation is marginal and makes only a couple of milli litres difference. Quite insignificant for bikes and cars. Maybe significant for buses and lorries

I would like to add one more important thing ....

Dont keep anything (like cloth etc) under the seat. It could stop the flow of air to the engine which, in turn will make a denser mix of fuel & air, hence gulping more fuel for the same kind of acceleration.

Also, proper tuning of bike is very important for a good mileage. the rpm during idling should be around 11 - 12 k. not more than that.

I have realised that its difficult to get the neutral while the engine is ON , especially during traffic stops. is it common with Dazzler?

he has meant to say this thing for small change in speeds...
for example,if your in 5th gear with speed of 45kmph and u reduce your speed by 5 to 6 kmph then there is no need for switching back down to 4th and then raising the throttle...but if your speed goes down to 35 or so then u may change gears...

while surfing i got this article which i thought to share with you guys..Aerodynamics:Gadgetry: Aids that draw power from the engine and battery naturally result in more consumption of fuel. This includes over size horns, additional headlights etc. Oversize and wider tyres also increase drag and reduce the fuel efficiency.

Highway Riding:Weight: Unnecessary accessories like excessively heavy crash guards, enveloping guards on scooters, oversize horns all add to the weight and consequently reduce fuel efficiency. On a 100kg bike even a 5kg increase will have an adverse effect on fuel consumption.

Fuel: Always buy fuel in the morning as it is sold by volume and expands during the day when it gets hot.

Cleanliness: Any leaks will be immediately noticeable on a clean bike and in a roundabout way will have an effect on the fuel efficiency. Dirt on the chain, in between moving parts etc will increase drag and consequently lower fuel efficiency.

Octane: Check the owners manual on what quality of petrol your bike requires. Modern bikes run a very high compression ratio and need high octane fuel (93 octane fuel is available in the metros) to run without knocking. Octane boosters (recommended on the Pulsar) also do a good job of controlling knock. However on engines designed to run on 83 octane petrol using higher octane rating petrol or octane boosters will have no effect other than lightening your wallet. Remember that higher the octane rating higher the price per litre.

Tanking Up: Do not tank up right to the brim. Not only is there a chance of spillage while filling up, petrol also expands with heat and can overflow. Filler caps on many bikes do leak once the rubber seal hardens and cracks. Tank up from busy and well know fuelling stations where quality and quantity are assured.

Engine Efficiency: Specific output is the most reliable indicator of engine efficiency. Specific output is defined as the ratio between power and displacement and higher the specific output in terms of PS/litre the more efficient the engine is. Lower emissions are also a sign of a more efficient engine. For example an engine designed to meet Euro II norms will be more efficient than a Euro I compliant engine. However an efficient engine will not necessarily translate into better fuel efficiency as it might be powering a heavier motorcycle and geared more for performance.

i rely on my bike's(zmr) RTMI..which proved to me quiet correct apart from that tank full method is universal..use it till last bar of your fuel meter starts blinking & calculate avg after subtracting reserve fuel from total amount of fuel filled & measure kms simply by trip meter..

Dude, is it true that in FI Bikes we should not run in Reserve? If so, Why?

they just have a low fuel indicator.
and they don't have fuel taps.

Hi, Fuel injected bikes rely on Fuel Pump , and these are inside the fuel tank mostly at the bottom of the tank.
1. Fuel in the tank acts as a coolant for the pump since it spins at a very high rpm.
If you run very low fuel below the recommended level the pump may overheat and may fail prematurely. Moreover fuel available is not clean every where and there may be sediments in the tank which may eventually be sucked in by the fuel pump and damage it. So that's why we should not run Fi bikes below reserve. Just my 70Rs.

yeah we should not run FI bikes in reserve continuously for a longer period because this may result in a damaged fuel pump..