At the latest press conference with Trump Hegseth stated that each of the two targets (Fordo and Natanz) were hit with 6 bombs.
Since each B-2 are reported as carrying 2 MOBs and seven were reported on the raid it would seem the seventh was an inflight spare in case any of the others had a problem en-route or over the target. With the small inventory of such weapons presumably they carried them home unused.
A bit of a bummer for the crew concerned.