Yeah, there's some variance, but within comprehensible ranges, and I agree, the ability to assess risk (both by PCs and DMs) is an important aspect of ensuring the progression of challenges is "fair". I used the term "smoothly" there perhaps a bit idiosyncratically - I mean they were predictable within a risk-reward calculus, usually within bounded ranges of difficulty that kept the worst options as extremely unlikely, had a clear ranking that was filled with options at each step, and there was an element of player influence over the bounded range (by choosing dungeon level and other factors).
One contrast I cut from the OSR / Classic discussion was to contrast the presentation of gods, which I think expresses some of the difference between the two cultures. Gods in Classic play, if they are not sublime beings outside of play who operate more as pure mechanisms of DM authority, are usually presented as particularly powerful monsters. You can, as a group of high level PCs, even expect to defeat them in combat. You wouldn't expect them to be thrown at you at low levels, but seeking them out and defeating them using only the rules-as-written is fine once you're powerful enough (tho' obviously still risky and dangerous).
In the OSR, gods tend not to operate in the same mechanical matrix as characters. They are traps, puzzles, and problems who are undefeatable in combat. One doesn't encounter them because one has surmounted level 20, but simply because their appearance in the game provides interesting, even spectacular, challenges to overcome (even if just survival). The challenges the gods pose encourage creative diegetic engagement, even in low level characters. Their incidence is unpredictable because they aren't easily slotted into a progressive set of challenges which are the the top level of, like in Classic play. This strikes me as far more "jagged" and "variable" in the difficulties they pose than the gods of Classic play.
Does that make sense?