Even some speedometers are not properly calibrated for speed, like take comparison of a bike and car and let both of them drive at 60 kmph on thier speedo, but in real there will be difference between them, i remember once at the local mechanic shop, I had to purchase some spare parts for my bike and took the mechanics bike and while driving his bike I observed speedo showing 40Kmph for real speed of 20 km ph, when i returned the bike asked the mechanic about it he told he modified the speedo to show high speed even though real speed is less to probably deter persons using his bike from over speeding.

I think it's done to provide a sense of performance to the owners. If 100cc commuter bikes start coming with a speedo with a top marking of say 100-110 km/h then people might feel "cheated" (don't ask me how specially when the primary concern of most Indian motorcycle buyers is to get a FE > 50 kmpl). However when the same bike has a speedo which shows 150 km/h as the top speed then the buyer might delude himself into thinking that he has got a great deal since not only does his bike give 50 km to the liter but can also touch 150 km/h when needed (though he'll never push the bike to that extent)

Almost all serious bikers know that the speedos are not accurate, they rarely show the actual speed while riding and the top speed is just a figment of imagination. I have a Bullet Electra with the top speed marked at 160 km/h. From experience I can tell you that you can push the Bullet to about 110-120 km/h tops, the only way I could do 160 was if a Hayabusa was pushing my Bull