That's something to elaborate in more details, not because I didn't specify it that it means it is impossible. It is more about some consensus they could follow as an ordering rule. Rather than each miners having their own rule, if they all agreed on, it would give this benefit.
Here is a simple one I thought about, but eventually that could be improved to better account for low-fees transactions that haven't been included yet:
Transactions are ordered first by
transaction fees (satoshis/bytes) (highest to lowest)
then by
transaction size (lowest to highest)
then, as an additional filter, the destination addresses of each transaction are XORed together and the lowest (or highest).
That consensus is hard to reach. Each node sees the unconfirmed transaction differently, they might even have their own rules which prevents some transactions from getting in. With that, they would still have different order in terms of the transaction arriving. For low resource nodes who limits their mempool to a very small amount, for any given block, they likely don't have every transaction in their "unconfirmed pool".
It is impossible for this rule to be enforced, since each node would have a different idea of what is the transactions that should be in the block.