When I submit a transaction how is it prioritized among same-costing transactions?

Example: When I pay 50 GWEI and 0 Priority Fee and etherscan shows average gas fee is well below that (say 40 GWEI) how is my transaction chosen/not chosen over transactions of the same cost for the next block?

I have noticed several times I've somehow "missed" low-gas blocks despite having a transaction queued for some time already.