As soon as you’ve spawned you will have a large mushroom biome in front of you. Continue straight ahead crossing over to the other edge of the mushroom biome and you will find the part of the biome which got some snow.
It’s obviously just the result of what can happen when a snow biome generates itself next to another biome. It’s definitely a nice and unique seed. The mushroom biome as a whole includes a large area and further out in the sea you can find some mushroom islands, so it got a varying a terrain.