If it was a glitch in their conversion from kilometers to miles, both would be off the same percentage. They are not. Also, bikes in metric counties have the same issue of optimistic speedos and more accurate odometers.
It is intentional and commonplace, not just with Suzukis, either. Some countries put stiff penalties on manufacturers if the speedo reads slower than actual speed, having operators unknowingly speed. So, they hedge their bet by making it read faster than actual speed and do a damn good job of making the odometer accurate - usually within 1%.
To answer kiley's Q - yes, you can drop a tooth up front with no change to the chain. Given, of course, to a chain in decent shape. If it's worn to the point it's near the end of the adjustment range, you probably can't. But then if the chain is that bad, it should be replaced along with the sprockets.