Why does making a Credit Card payment result in funds in "Ready to Assign"?

I've watched the Nick True video that came highly recommended in a bunch of Reddit results (e.g. here), but I still don't understand what's going on.

1. Have all funds assigned, so that "Ready To Assign" is at zero.
2. Approve a(n automatically imported) transfer from my checking account to my credit card account (see screenshot). Note that it is registered as an inter-account Payment and not as a regular expense.
3. Budget view shows an equivalent amount of money "Ready To Assign".