My understanding is that it is reliable but not fully predictable in the sense that you can't guarantee that something between 1.5 and 3V will lead to the same HIGH or LOW on two different boards or that the same input will lead to the same HIGH or LOW if you power supply has somewhat varied between the two reads. But that's just a guess, I've not studied in details how the PORT registers are set electrically
Not weird, it's part of the language, so something good to know 