Welcome to the forum,
I no longer live in the North East / Mid Atlantic area but have experienced years when mayfly hatches are definitely out of sync from the norm. Given the rather precise life-cycle of certain species I suspect that at times the hatches either occur at a more punctuated rate or we simply are not present when these occur. It seems that many animals who dwell in aquatic environments have certain and differing levels of variance built into their behavior patterns as a sort of fail safe. Such modifications act to preserve the species to some extent.
For instance, although speaking on a different subject; salmon that have schooled at the mouth of a river although ready to begin the spawning migration may hold off from entering during periods of low and warm water conditions. This can only last for so long before the maturation of the eggs and milt being prompted by hormonal activities in the fish must be answered but it does at times hold off a run. Getting back to mayflies, these members of the animal kingdom are reliant upon a rather narrow window of conditions for optimum reproductive activity also. The hatches will come albeit perhaps late and without said optimum conditions less the regional population be in jeopardy for the future.
I am not a student of entomology but have spent a lot of time on rivers, creeks, and streams.
Glad to have you join,