The IRS accepts most statements made regarding facts on return unless there is clear error, in particular because refunds are due (per statute) much faster than the IRS is able to gather all relevant information on whether it is factually owed or not in some edge cases.

The backstopping function is that stealing from the IRS is a really bad idea because you'll probably be interacting with them the rest of your life and because you probably want access to the US financial system at some point.