Easy, simple. Between slight head winds, going slightly uphill or downhill (it may LOOK flat), whether a big boxy vehicle next to you is creating an air pocket, and atmospheric pressure will factor in how fast or slow you go. The speedos aren't the most accurate in the world either.
I have a KTM Foxi ( the one that looks more like a Puch)
It has a CEV speedometer that reads about 7 mph fast, so when it says I'm going 35 I know I'm really going 28. If I get a good combo of speed factors I can push the needle up to 40, but I know there's no way an untuned Sachs can go that fast.
Spartas got a little more back, they weigh more so you'll never have great takeoff or hill power. But if you get things all just right they can go about 33 without swapping a single part