(BTW, I've always wondered whether, given that the conjunction is on the list as an "alternative" choice, subjects interpret "X" by itself as "X but not Y". I've always thought someone would have done experiments to test that idea, but I haven't looked into the literature deeply enough to know.)

Actually, even though this explanation works for the oft-cited Linda case, it turns out that the locus classicus Kahneman & Tversky 1983 already contains several (versions of) experiments that yield conjunction-fallacy-type results but could not plausibly be interpreted in that way.