Manually (as in manus, the hand) measuring your pulse while not at rest (sitting down, in a calm environment) is so error prone, it's almost useless.

The only proper comparison would be with a chest strep, which are much more accurate.

Also, it really differs per person and how you wear the watch. Just wearing it a bit differently can change the results significantly.

Sweat also gets them messed up easily.

All reasons why, if you want accurate data for workouts, you should always use a chest strep. Even if they are much more awkward.