RE: "All it knows is that you're trying to write a 6. If someone else wrote a 6 or a 7 in the meantime, then your transaction may have 'meant' (+0) or (-1)."

This is not how it works at all. This is called dirty writes and is by default prevented by ACID compliant databases, no matter the isolation level. The second transaction commit will be rejected by the transaction manager.

Even if you start a transaction from your application, it does not change this still.