I don’t think there’s anything to be made from the disparity other than the fact that in summer you have tremendous available moisture much of the time. So when it does rain, it can rain a lot, quickly. If you happen to get it, you get a lot. When you don’t, you don’t. It’s pretty much luck of the draw.
Being on the lee side of the mountains out here probably hurts some, but by and large I think it’s just luck or lack thereof.