Why does line 2a on form 1040 not match the number I input for tax-exempt interest?

It can be somewhat lower if you entered some $$ on line 13 of a form 1099-INT...or are reporting "Accrued interest " on a municipal bond you bought last year.

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But if higher than you expect......

.....Just to check.....line 2a includes both box 8 from all your 1099-INT forms,  AND box 11 from all your 1099-DIV forms, (also Sometimes info from some K-1 Forms if you entered any).  Recheck all those first. 

IF it still doesn't add up properly, you may either have a duplicate -INT or -DIV form you need to delete..or worst case, you may have to Delete your entire Schedule B  , and re-enter all your 1099-INT, -DIV and -OID forms.