Depth is not rocket surgery, but some judges make it out to be. The top of the thigh where it inserts at the hip has to get below the top of the knee. This is an objective way to determine depth. A “full squat” would not be, as fatter guys would not have to go as low, leverages would limit depth, etc. This is why powerlifting chose the rule it did. All that remains to be determined by the judges is whether point A gets below point B. Since the rulebook does not specify by how much, any value greater than zero is legal by nearly any powerlifting federation’s rules (including the APF, IPF, etc.).
So there should be no difference in interpretation, however some judges execute the rules differently. Sitting back and saying “he was close enough; I’ll give him the benefit of the doubt” when the dude is an inch high is poor judging. What others don’t realize is that sitting back and saying “he did not convince me enough that he got below parallel” when the lifter is an inch below is also poor judging. It is not strict judging at all - because strict judging is following the rulebook exactly and not making up your own standards. If you need a lifter to go 6 inches below parallel to let you know that he got below, then you are a horrible judge and need to be taken out of the judge’s chair (and most likely have your driver’s license revoked for your vision defect as well). This is no different than a football referee saying, “Well, since he didn’t get the football 3 yard into the endzone, I can’t guarantee that he broke the plane, so we’ll spot the ball on the 1 yard line.” That is reprehensible judging.
That being said, getting depth is a problem for some people. My suggestion is to find a stance that is comfortable and powerful that puts you convincingly deep for even a prick judge. Train with that stance most of the time. For most people this will be more of a moderate stance and not that wide. It becomes a lot easier to get depth as you bring your stance in (just don’t shoot your knees forward too much to compensate).