I'm in the "not likely accurate readout" camp. I have a Garmin and it gives me super high readings once in awhile. I figure my max HR is around 185. My usual high up the dreaded hill is round 180 but sometimes it shoots up to 225 or so when I fly down that hill. It's at the very beginning of my ride so I think the thing just isn't "set" quite yet and although I'm sure HR does shoot up high when I'm doing 60+ kmh, I'm positive it didn't really go up to 225.

If I were you, I'd be more worried if it stayed at that high rate over a period of time. It would be interesting to stop and take it manually to see if it really was that high...