That assumes the authorization levels stay constant. They could have tightened down the inventory.

Also, some lazy agents don't like to check, so if they see Y > 0, they say it's not over just to get you off the phone.

Finally, the term "oversold" or "oversell" refers to actually needing volunteers (# pax checked in > # seats). From a technical sense, they won't know if the flight is oversold until minutes before departure. You should be asking if the flight is "overbooked."