The
Airborne strongpoints are indeed very weak. I picture them not as bunkers, but as local squads of defenders assigned to man checkpoints and block roads in case of invasion.
Quite a few of the Normandy defenders were Ost (East) Battalions -- POW's from the Eastern Front given the choice of enlisting or going to a camp. They were notoriously unreliable. The 'unoccupied' strongpoints are not necessarily dummies, but can represent checkpoints abandoned when the shooting started.
See:
Ost Battalion Reenactors