GK will eventually gain bad form if he is unchallenged (no shots against him) all the time.
Using GK with bad form is quite dangerous, I say a form 6 GK has less than 50% chance of saving a shot, a 5 would be less than 10%.

A player form is the major deciding factor the chance it will complete any task. If there is no task the form means nothing. Your defenders stopped all shots and others were out of target, leaving your GK with nothing to do.

Not all shots could be saved by a GK, some shots are unstoppable by the GK, instead only by defenders.

His GK must have many more saves compared to yours.