Where do I have the math wrong?
$3,600 Total credit available (based on assumption of MFJ return, under the AGI limits & no kids)
-3,000 Total EIP amounts received ($2,400 + $600)
$ 600 Rebate recovery credit available on return
If the software is showing a $1,200 credit available, I'd dig into the math and see which one of the above numbers was different.