"a history lesson" and "an historic event" have different stress in the history area (or at least they can have, depending on the phrase context), and I think that explains it.

Saying "a historic event" requires you to pause and stress the start of historic (a + glottal stop + "his" + ...). It's a natural phrasing pattern to have the initial part of historic unstressed, and then the h is effectively dropped even if you're not speaking an h-dropping dialect; this way "an historic event" follows as correct.

We simplify a lot of sounds in unstressed parts of words, but because we know and "see" the spelling, our brains can keep the illusion of the whole word going for us.