I know it's kinda unusual that the "blockchain" fee that he should been deducted was returned to his balance.
Perhaps there was some changes when he was withdrawing, or some error on Roobet's withrawal system. But I guess it's more on a sudden change of tx fee.
Yes. Now you get it. That is the important part.
I say this because I had a similar experience months ago (I got some money back in my account balance despite having requested a max withdrawal) and I'm sure I recently read something similar from another member but I'm too lazy to search for that post right now. Perhaps you are right. It's possible that the recommended fee changes at the time of withdrawal, and Roobet doesn't take this into account, but returns the rest of the amount to the user. This is most likely a rare issue that is hard to replicate.