I hadn't read the bottom part of your post and came to a decision about literally the same thing you did. Localized weather, rapidly changes, some monsters later can effect it in an area, but for the most part the biome of the floor depicts the weather.
So one of the floors is guaranteed to have heavy downpour all the time while another could be the desert. Stupid hot all day, cold as my heart the rest of the time.
Good to see we're on the same page XD