All these numbers are the numbers shown online on the online sales system.
It's quite a big assumption to make that all the other seats have sold. Almost certainly there are single seats in L1 and L2 that are unsold, and there maybe tickets that for some reason unknown to us are not on the plan, for example those given to third parties, and there's also hospitality seating such as the Tunnel club.
In other words what is being counted is the minimum of seats left (1,937 at last count).
You would think that if City had seats to sell in Level One and Two then they'd be trying to sell them in preference to those in Level Three so you can forecast a crowd of 50k but you can't know for sure.