Extreme Hills having both snow and rain?

I was expecting Extreme Hills biomes to have just snow and not rain (since it now snows in that biome).

What actually happened was that I saw snow in parts of the Extreme Hills, and other parts rain. I went into F3 mode to make sure it was in Extreme Hills

I guess to reproduce this, you would just reproduce a world and find an Extreme Hills biome, and see if it is both snowing and raining in that biome.