Inaccurate (higher than normal) heart rate after a run

Sometime I notice that the HR is higher than normal (100 vs 80)  even after 1 hour rest.

I compared it with chest strap and other HRM device and confirmed that garmin's reading is abnormal.

If I immediately take off the watch from my wrist, re-wear it after 5 seconds, the reading is back to normal.

This happens for several times.