be issues with your clutch (/gearbox if auto), tyre size if you've recently fitted new ones, or your revcounter itself... or maybe you've found a new top gear?!
TBH it's most likely that you're imagining it though!
Clutch/gearbox issues would be obvious in other ways, but there aren't many things between the engine & the wheels which can affect the relationship between how fast they turn! It's like saying (on your bike) that you've found you don't have to pedal as fast to go the same speed in the same gear...
Ride slow for FeAR of FLYiNG PAST dReAMS
Fat Buckshaver, SC Heckler, Prof 856, Orange P7