It appears that the Burning Crusade Classic Auction House API reports invalid auction data that presumably includes cancelled and/or expired auctions.
I have observed the following auction counts in the Horde Auction House on Venoxis (EU):
Time | #Auctions In-Game | #Auctions API
2022-05-02, 17:32 CEST | 85,205 | 103,474
2022-05-02, 18:19 CEST | 85,903 | 109,617
2022-05-03, 23:28 CEST | 87,278 | 117,067
The API reports consistently more auctions than there are live auctions in-game. The snapshot nature of the auctions provided by the API does not readily explain the significant discrepancy.
A similar issue has been discussed in the past: