There may be all sorts of reasons HRM give wrong readings.
Poor placement of the chest (sensor) strap
Poor electrical contact (too lose)
Issues with the equipment.
Have you compared your resting HRM reading with your pulse?
If there is a difference this suggests some sort of technical problem.
Heart rats can be a measure of how fit you are as you get fitter your heart rate will be less for the same type of exercise.
Reminds me of the time my father was in hospital with heart issues.
He was snoozing and the heart monitor was flat line I was getting a bit emotional before I realised the leads were not connected.
