We have 2 identical transactions, one spends 22409, and the other 37345 gas, why so?
https://etherscan.io/tx/0xaeefbb8aab4fd61fc057b4013f8ca2accd4ed573ec4dcfacc02a1bcfb99cb3f8
https://etherscan.io/tx/0x42071ede01368cd04c621ccbc2197da91302a54da320ee03147bd86e3742cb02
The difference of ~15,000 makes me believe the difference is in the balances before hand. A SSTORE operation (change to a storage slot) in the EVM has different costs based on the value pre-execution and post-execution. If the pre-value is 0 and the post-value is non-zero, it costs 20k. If the pre-value is non-0, and the post-value is 0 it costs 5k and a 15k refund is added to the refund balance. If the pre-value is non-0 and the post-value is non-0, it costs 5k.
The transaction that cost ~22k is likely one where the sender and receiver have non-0 balances before, and the sender has a 0 balance after. So there is a 5k+5k charge for updating both balances, and a 15k refund for the zeroing of the sender balance.
The transaction that cost ~37k is likely one where both have a non-zero pre and post balance.
EDIT: You can actually see these to be accurate if you click the State Changes tab on the transactions on Etherscan.
