I think it is a government regulation. I have checked the speed against cars and had them run exactly 70 and get behind them.....and I read exactly 70. Seems to be an universal regulation for all vehicles. Cars...bikes......all of them. Checked against the Zumo it is about 3 mph slow.....as is my 2001 jeep cherokee. Kit
I don't think the speedo error is linear. At 30-45 mph range it appears to be 4mph slower than indicated judging from the roadside radar trailers. I recently went through a radar display sign on the interstate at 75mph and it indicated my speed was 75mph. Up to that point I assumed the error was linear.