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Generally speaking, on a cheapo scooter you have a mechanical speedometer, where the needle position is based off of a spinning cable.
These types of speedometers are built to a specific tire size and speedometer ring, and generally can't be calibrated. Wherever they are when they leave the manufacturer, is where they'll be. They are susceptible to changes in temperature, too.
On a more ... expensive bike, especially the ones equipped with ABS, the speedometer (as well as ABS) is derived from the tone wheel attached to the front wheel; this is an entirely electric system, with a variable reluctance sensor placed near the tone wheel, which feeds into the ABS unit (and the ABS unit subsequently provides electronic pulses to whatever is controlling the speedometer, typically the ECM). These bikes are pretty close to spot on, as the only thing that requires calibration is the ECM / speedometer pair, which is typically done by the manufacturer before the bike leaves the assembly plant, AND the manufacturer has impetus (brand name protection) to actually have the calibration done.
I've had a Kymco Xciting 500 ABS, which @ 70mph GPS was showing 72mph (3% error). I also own a Honda Forza ABS, which @ 71mph GPS shows 70mph (1.5% error). These were worst case readings over a variety of speeds.
None of the scooters with mechanical speedometers I've ever owned have ever had a worst case error of <11%. You generally need to mount a GPS to the bike and learn how far off the speedometer is from the real speed. I honestly can't blame them for not allowing user calibration, you can bet there would be schmucks who would screw around with it before selling the bike.
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