I have a pretty old Garmin HRM Run sensor and a newer Polar HRM H10. I used Polar most of the time connecting it to Garmin Edge 830, but recently the strap worn out and I decided to use the Garmin HRM. I had to replace the battery because the old one died. After using the Garmin HRM I noticed that its measurements are way off. On a certain ride it counted an average HR of 173 with maximum being 220. This is highly unusual because Polar usually gives me something like 144/163 BPM. Now I am not sure - is the old Garmin broken?
My old Garmin HRM was prone to spikes (but I am going back 8 or 10 years to when I last used it), my H10 never spikes but as its aged it has a tendency to read low.