Why would a change in venue change the amount of experience necessary to raise in level?
Although the number of XP needed is constant, the rate of acquisition can vary.
By 1987, not only had TSR published many more modules but I had more money to spend on them. I did not find so much worth the purchase, for the most part sharing the estimation of "classics" you mentioned.
My formative experience, though, was one in which modules did not figure as a (much less THE) staple. I have noted some contributing factors.
There is, I think, a notable correspondence with the number of computer games in BASIC that I either transcribed from magazines or wrote for myself (the Commodore 64 having been a delight from what must seem now nearly a primeval perspective).
It was another time, and even at that time things were different in other places. A friend of mine, born into a richer house, had full runs of The Dragon and Different Worlds, and seemingly every module from TSR as well as a good few from Judges Guild.
On the other hand, he was stuck with a TRS-80 on which to play Temple of Apshai ...
