No way to know in this case, but I agree that there are some merges rejected (or set to match not merge) which should be accepted as matches. Small variations in spelling, a single year off on birth or one has a correct date but the other is "before" or "after", irregularities in geographic locations. Sometimes it's because they have different spouses or children, which doesn't hold water since it is fairly common for people to have more than one spouse or child.
A common cause of birth discrepancy, for example, is when one profile has the birth date and the other a baptism date, especially when the baptism was separated from the birth by a matter of years, like when several siblings are baptized in a batch.
Sometimes everything matches but one thing. When one is sourced and the other isn't, I go with the sourced data. We sure do have lots of unsourced profiles!