There are a number of subtractions that can be included on line 13 of Form 502, not just income from U.S. government obligations.
Line 13 of Form 502 is populated by the total from line 1 of Form 502SU, Subtractions from Income. You can see in the linked Form 502SU all possible subtractions that might be carried over to line 13 of Form 502.
