Ahh, I suspected 3E worked like Pathfinder, but I wasn't certain.3rd edition specifically says that even if you have an expanded crit range larger just the '20', only the '20' is an automatic hit. Other numbers in the expanded range have to actually still hit the target's AC to qualify for the possibility of being a "critical hit".
Now granted, in 3rd edition... rolling a 20 or a number within an extended crit range does not in itself generate a "critical hit"... you merely threaten a critical hit. If you made that first roll, you then had to roll a second time and if the second roll would have also qualified as hitting the target's AC, then the threat was confirmed, and it became a true "critical hit".
I believe others have stated earlier in the thread that because a 20 was not considered a "critical hit" straight away in 3rd edition, but merely a threat of a critical hit... that is the reason why we didn't have the idea that auto-hit = critical hit. But since in 5E a roll of 20 is called out as an auto-hit and in addition a critical hit, that is why some folks have determined this to mean that in every case auto-hit = critical hit.
A valid way to look at it... but not definitively the only way to look at it, in my opinion.
I should clarify a couple points about where I'm coming from, because there's a surprising amount of nuance going on here. First, I agree that the general rules suggest a distinction between an auto-hit and a critical. This seems designed to leave the door open for an attack that could have missed dealing critical hit damage (ie., a feat/feature/spell reading "if the attack hits, it is a critical hit").
I just don't think that's the case for Improved Critical, because of the way it's phrased (underpinned by the specific-beats-general axiom). This leads to my second point: "critical hit" isn't the only important part of the wording. Improve Critical says attacks "score a critical hit." It strikes me as strange to score a critical hit, but not hit and not gain the benefits of a critical hit, especially since the Critical hits section uses identical language ("When you score a critical hit...").
I might find that interpretation more relateable if there were some sort of precedent - if "score" or "score a critical hit" could also be considered terms of art.