Is there a reason why I can't get my full 5GB back in iCloud?

4.9 may be 4.95 or even 4.999 rounded (or just a few MBs). Very likely that info related to your account is stored and/or formatting reasons or maybe just reading errors account for this super small amount. Are you really in need of this last 1MB to 100MB or just OCD?

I don't think you're going to get any more free. 4.9gb is probably so close to 5.0gb. There may be system files/folders needed to be inlace that is occupying some space that is needed for iCloud that you cannot get rid of.

I have the same 4.9...It's due to the original default location on a clean Mac installation being the cloud, instead of the location you wish the document / item to be placed.

In my case it was a fairly short Pages doc...I deleted it, but the cloud storage remains at 4.9

I have other things to keep me awake at night..

Email, documents, preferences, etc can all take up those few hundred Kb.

I don't think you'll get to the 5gb as I mentioned, but I'd personally like to know what it is