It's possible that the lender removed the reporting of the 30 day late. Whether it was removed intentionally or by accident, don't ask don't tell... no harm no foul. Just be happy it's gone and hope it doesn't return.
As for your score, the gain you receive would depend on the presence of other negative items. If you had an otherwise clean report and the 30 day late was the only negative item, removing it would cause a score increase. The amount would depend on the age of the late payment. If it was recent, you would stand to gain quite a bit. If it's older like yours was at 4-5 years, most of the sting from a 30 day late had already worn off, so you wouldn't stand to gain a significant amount. If you have other negative items on your credit report, which it sounds like you do based on the negative reason codes, the removal of a baddie often results in little to no score gain.
Keep in mind that monitoring software like CK only looks for late payments in the last 2 years, so lates older than that aren't shown in their fluff software charts/tables. This would lead many to believe that they don't matter after 2 years, both under FICO scoring and VS 3.0, when in fact they do count much longer than that. You already know this though, as you referenced the 7 years above.
