As you might know, you can use several filters in Scanner->Set Information to scan only *some* sets of a dat.
There are some reasons why this limiting is a good idea, but people should be aware of sideeffects:
A user wondered if there were sets removed from MAME 133 to 133u1.
After some discussion he mentioned that he only has some sets enabled.
Well, there's the answer to his problem. He did not scan all sets.
He did limit the known sets by disabling them (by only enabling some) in setinfo.
This means the 'new names' of u1 are unknown and so the wrong named sets are listed as unneeded.
Limiting sets is not a good idea anyway. Why do users want to limit them? Because they don't want to see 19247394714 missing set information if they only have 3 sets.
Solution: Keep all sets enabled!!! Hide fully missing set messages via Scanner->View (popupmenu)->Hide Fully Missing Sets.