Weird HRM readings, Polar rs300x

MooseWizard

Just finished my first ever outside run. Have done the Couch to 5K program, but only on a treadmill prior today.

I have been using a Polar rs300x with foodpod for several weeks. It allows you to enter your VO2Max, or has a built-in test to determine it for you. I have read this is fairly accurate for this HRM, and had used the test to update the settings of the watch. It estimated my HR max at 186. For my age, this is very much in line with other online calculators and formulas I have seen to determine HR max.

Now for the weird part: Part of my run today, in had measured my heartrate at 190 bpm, or 102% of my HR max! Max is max, so this is obviously not possible. My question is, which part is off? Is the heartrate not measuring correctly, or is it possible that I fall in that 5% of people for whom the formula's for HR max is not accurate? For my run, it also had my average HR at 180, or 97%. There is no way I am fit enough to run 33 minutes at 97% of HR max!

As I said, I have been using this HRM for several weeks, and not seen anything like this before. It also likely undershot my calories burned by about 200--especially if I was actually working as hard as it says I was. Anyone else had weird readings like this?

Distance was also off (as confirmed by GPS) but that probably just means I need to calibrate the footpod.