Strange - I've been going back and forth between overextension and not, monthly, a few times now, without my number of provinces or my number of cores changing. As I understand it, my overextension-or-not status shouldn't change if neither of those things change.
I'm at almost exactly 50% cores (I haven't counted, but that's what the ledger graph looks like), and "less than 50%" is the cutoff for a leader with the admin skill of mine (7), at least according to the "triggered modifiers" window. Maybe I'm stuck in a weird rounding error, where the computer sometimes decides that (say) 49/98 is 50% but sometimes it's 49.99999%. Maybe on different processors of my computer or something? Seems like it should calculate the same result consistently, and regardless of the processor, but I don't see why else I'd be flipping back and forth without my number of provinces or number of cores changing.
Well on some computers, sometimes 2 + 2 = 4.02. So you might be right.