It makes sense but I disagree with you. Board is 5 6 7 8 hearts Q spades. You have 4h 4d. You re-raise and your oppo pushes over the top. Does he have a straight, a flush, the nut flush the nine of hearts? The latter may not be likely but it's far from impossible.

So while straight flushes are very very rare, it's not all *that* rare to have two straight flushes competing when the board makes it possible.