When entering information about an alma mater of a person the source that gives the start and end dates of attendence don't have to be the same. The thesis might again have another reference. Currently it's not possible to specify exactly which reference is responsible for what qualifier. To me that seems bad, because it makes it less clear what the references support and what they don't.
I've usually referenced the statement to two sources in that case. I agree it's not ideal, but I would guess it's a bit of work to associate qualifier to source (and the UI implications? :D).