see: https://docs.web3js.org/guides/transactions/transactions/#step-4-send-a-transaction-and-review-the-results:~:text=Note%20that%20the,by%20effectiveGasPrice).

effectiveGasPrice is the price per gas at the time of your transaction, so the total gas cost of your transaction is effectiveGasPrice * gasUsed.
gasUsed is the total amount of gas used by this specific transaction.
So, why use cumulativeGasUsed instead of gasUsed for calculations? 😕