While it may not be impossible to to rebucketing, it's extremely rare for the removal of a negative item/account to result in a score drop. I would suggest that the drop came from something else, but would like to hear what other opinions are on this
FWIW - The Experian score used for mortgages is Fico 98 (Experian Fico score 2). It is true that 3B reports do include Experian Fico 04 (Experian Fico score 3) but, the mortgage standard is Experian Fico 98.
In contrast, mortgage lenders have standardized on Equifax Fico 04 and TransUnion Fico 04.