Another explanation could be you were using more slots than they offered you before you upgraded.
For example, you had 50 slots, but the airport only allowed you 35 (because your service quality dropped after the initial 50 or other factors).
After you upgraded, it now allows 48 slots, but you're already using 50, so you didn't gain any "new" slots.
This would be quite difficult to happen tho, the airport would have to be a really busy one with few slots for this thigh margins as in my example to be able to appear.
Loyalty is everything tho, that b* airport is probably cheating on you with another airline.