Please Explain iCloud on iMac Versus iPhone/iPad

I have 200 GB of iCloud storage. When I look on my iMac it says that 115 GB is used. When I Iook on my iPhone and iPad they both say that 191.1 GB of iCloud is being used. On those 2 devices it looks like a lot of that is for Back Ups. If iCloud is iCloud why does my iMac not show the same amount of space being consumed?

Thanks again to all of you smart Apple People.