Not likely to be a mistake. Although we only have the TU score to go by, it sounds like the effect of a scorecard reassignment -- while apparently rare, it does happen. The gist is you've been moved to a new FICO scorecard now that your last major derog is gone. So, instead of being compared to others with major & minor derogs to determine where you fall on the scoring scale, you are now being compared to others with no major derogs and only minor derogs to determine where you stand scorewise. On the new scorecard, because your lates are fairly recent being only 3 years old and there are quite a few of them, you may fall within a lower range, sort to speak, within that scorecard against people with more aged and / or less lates - so your scores are also lower. The good thing is on this new scorecard, your potential score ceiling is higher than it was on the old one. Your scores will rebound with time.
You can search the forums for more detailed info on "scorecard assignments / reassignments" aka "rebucketing" to learn more. Will be interesting to know what happens to your other 2 scores when this account is removed.
I experienced the same when my last derog was removed early; I was left with no minor delinqencies / major derogs, a completely clean report, but lost 24+ points accross all 3 bureaus -- the only thing that changed was the removal of the derogatory account. The reasons (i believe) I may have fallen on a lower tier on my clean scorecard is my low age of accounts and the number of new accounts / inquiries I had acquired just 1 month before the removal of a 5yr. old paid charge-off. Seems I am penalized much more for inquiries and new accounts now than I was when I had a major derog. Also, my loan balances are now considered a negative factor and my scores are much more sensitive to utilization than they were before. The only thing I can do at this point is let my accounts age so my scores rebound.
I know you were hoping for a higher score for another mortgage -- but, on the upside, a cleaner report with lower scores is still better than having a major derogatory.
Others may eventually chime in with more questions and possible causes - but that's my theory if you are sure no new derogatory information has appeared on your reports since the removal of this charge-off.
