Just like you topic says, it just an estimation and not the exact number of blocks in which your transaction will get confirmed. The state of unconfirmed transactions in the mempool keeps changing as new transactions with higher fees keep coming in and the time taken to confirm the next bitcoin blocks is something rather random. So, If the next block takes over 30 minutes to get confirmed then be sure you transaction has been pushed a little further away from the mempool tip than it was 30 minutes ago when you broadcast the transaction.

I suggest you use the "In the next block" option next time, again it's not a guarantee but still your transaction stands a higher chance of getting confirmed quicker

I prefer the Mempool fees estimation option along side a couple of tools like mempool.space. They give a better picture of the mempool in real time unlike ETA which most bases on the previous block