I would think that it almost has to be a measurement problem. I once did an average speed of 115 mph on a C ride. We must have had to slow down for the STOP signs. Electronic apparatus can work great but when it malfunctions, it can be way off.

On my polar, I get readings that are double my actual heart rate at times. You could have been running something like 106 at one time and had it read 212.

I have hit as high as 196 but that took eye popping effort. Anything over 170 takes are real hard push. So I would think that if you had pushed hard enough to get that reading, you would know it because your tongue would be catching in the spokes of your front wheel.