It's hard to reason accurately about unusual situations, in poker as in life.
One reason for this is that it's much harder to draw on your experience to figure out what's going on. Another obstacle, however, is that usual ways of analyzing hands become far more difficult. Here I'll describe a simple but effective way to modify one such procedure in unusual spots.
Specifically, I'll talk about counting combinations, which is a good way to figure out your opponent's range, but which gets messy as ranges get very wide or very narrow. When almost every explanation for an opponent's play is unlikely, you wind up trying to add up sets of fractions in your head (which isn't easy to do accurately over the felt, at least for me), or else you risk making rounding errors that are large relative to the quantities with which you're dealing.