Every time you add something to a paycheck, the paycheck re-calculates. So, in your case, for some reason the result is the same after recalculation.
If the additional items you mention are additions, are they listed on the other table on the paycheck above the 401k or after? If after, then their amounts aren't included in the deduction's calculation. In this case, if you put them before the 401k, then does the item update as expected?