I don't know why this behavior is still a thing but this is the way the layer mixer works.
With the understanding the nodes in the Color page work strictly from left to right, you would think that the shot of the building would be on top and the cartoon dragon would be on the bottom but that's not the case. For some reason the layer mixer's inputs show the bottom-most layer on top and vice-versa.
I know this behavior's been around for awhile but that doesn't mean it makes any more sense.