Thanks for reporting this!
The discrepancy is probably due to the output occurring in multiple blocks on different forks at different heights. As @maksym showed here, there can be many forks on Testnet4.
It is possible that blockchain explorers and the Bitcoin canister occasionally see a slightly different state when there are several forks and temporarily disagree on what blocks are part of the main chain.
As you observed, once there is agreement on the main chain, the discrepancy goes away and all sources report the same information.
Unfortunately, there is no way to guarantee that this never happens.
I hope this helps!