Hah.
I'm sure it has to do with periodization design and is probably highly individual in scale.
For example, if I want to have good legs for a long strenuous ride on Sunday, I head out for a single-speed low cadence grunt day on Thurs. eve or Fri. am. And it has to be a grunt muscle-up day, not a spin day. For whatever reason, on Sunday it reduces the evident fatigue from Fri eve or Saturday. Same thing with swims - an Indian club session Sun eve or Mon makes a noticeable, positive, difference in swim feel on Weds.
I'm sure there's a dozen ways to make "you're compensating intensity for ___ on the intermediate day" arguments but the pattern is there whether or not I actually miss intermediate day.
I'm sure I'm not making any body changes whatsoever on so small a time scale. But it's there and it's weird. I think it might be sort of related to something
@Tony S was talking about in a different conditioning thread - how the first long ride / century of the summer tends to turn a corner and make everything just that little bit easier in the weeks just after it. Mental? maybe.
I have yet to figure out how to game it for downhill skiing, tho.