The problem is connecting data storage and iCloud data

I think that when counting files, they are forced into specific pre-determined bins, and some files might not fit or they may be a bit ambiguous. After all, aren't some pictures really documents? Do thumbnails and previews count as pictures, or are they system things? How about icons and logos?  And Macs and iPhones and iCloud have different apps and different sorts of files, so I'd expect the numbers to be different there, too.

I have no special knowledge, but I'm not too surprised that different systems count things differently, and some things may get counted twice. As you pointed out, the numbers are dynamic, constantly changing, and "it resolved itself six months after it occurred."

As for space on your computer, using iCloud doesn't always help. iCloud's major purpose is to make files available on your Mac, on your iPhone, and on your iPad, so you can work from any platform. It tries to keep files on the device for quick access, so you're not really offloading the files to a different place.

I took Russian in University, and all I can say is

Я иду на почту

My suggestion is buy yourself a dependable SSD or HD that you can back your data up to. iCloud is not a backup solution, it is a synchronization service.