Perhaps rules on this detailed subject are difficult to find.
I know. This is the typical case where one has to find the rule by comparing examples. My first question is, why is this sentence incorrect?:
a. The apples are fresher than were bought yesterday.
I think it has to do with the lack of identity between the current apples and the ones bought yesterday.
My first impression is that, for "as" and "than" to work in reduced clauses, there must be actual or potential identity between the two things being compared.
b. lf you are overqualified for a particular job, you have more experience or training than (is) needed. (We are speaking about the same experience or training, whether actually possessed or potentially required.)
c. The results of the treatment will be less positive than would be achieved if aspirin were added. (We are speaking about the same treatment results, whether actually or potentially obtained.)
My second impression is that this reduction seems to work at all times when the clause refers back to the whole previous clause. In this case the verb "be" (if used in the reduced clause) will be in the singular, because reference is being made to some situation:
d. The students were better than (was) expected.
e. The results were as fine as (was) expected.