Pump guns depend on manual operation to eject, and how forceful the gun ejects depends almost entirely on how forcefully you operate the gun.
Ease the action open gently and the shell may not eject at all, or it may just sort of dribble out.
This is no different then many firearms, especially pistols. Ease the slide open on an automatic and it won't eject well either.
However, other then ejection, you don't have to operate an 870 with great force for it to work correctly. Using "some" forceful operation is useful to insure you don't accidentally "short stroke" the gun and cause a failure to feed, and as you've discovered, it will insure proper ejection.