That is just about the perfect empirical evidence of a weak FP spring. Weak springs will cause inconsistent ignition. Some will fire, some won't. The fact that more fire than don't isn't at all surprising.
The other, even more common cause of inconsistent ignition is seating depth as many have said, and you have addressed. However, without being able to eyeball the actual seating depth of the primers in your cases, it is hard to say if they are seated properly or not. You measured them for consistency of depth, but apparently not for actual depth. Can you determine how far below flush they are for comparison purposes?