An IRR comes from the traditional account in your 401(k), not from your paycheck. The contributions from your paycheck go directly to either your traditional or designated Roth account in the 401(k).
Regular elective deferrals to your traditional account in the 401(k) are pre-tax, not after-tax. If you had any basis in after-tax contributions to the traditional account in the 401(k), that would have been reflected in an amount in box 5 of the Form 1099-R and would have corresponding reduced the amount that the payer reported in box 2a. If the amount in box 2a is the same as in box 1, box 2b Taxable amount not determined is not marked and there is no amount shown in box 5, the plan believes that you have made no after-tax contributions and you probably should believe that too.
