@fanfare
I think its because the instructions for line 13 dont specifically say "as reduced by excluded sources of income see definitions). The instructions just say
Line 13—Modified Adjusted Gross
Income (MAGI)
If you didn’t exclude any amounts from
your gross income under section 911 and
you don’t own a CFC or PFIC, your MAGI
is your AGI as reported on Form 1040 or
1040-SR. If you exclude amounts under
section 911 or own certain CFCs or
PFICs, your MAGI is your AGI as modified
by certain rules described in Regulations
section 1.1411-10(e)(1)
When reading the instructions for line 13 you have to remember the list of items at start of the instructions for excluded income. If you didnt do that, you end up just using AGI which is incorrect.
