Possibility exists the Enterprise was involved with some other combat situation on her way back to Earth? They had to pick up the Reliant crew then ferry Saavik and David Marcus to the Grissom.
Yeah, this is my reasoning too.

Although Kirk is clearly still mourning the loss of Spock, there are other indicators that events in TSFS might not be exactly as close to the end of TWOK as we are otherwise lead to assume.
I also understand that making movies years apart that are only supposed to be days or weeks in movie time can present especially hard problems in this area.
I sorta kinda have a theory behind this as well: we forget, in the age after home video, that the idea of being able to watch movies and their sequels in quick succession was not always available to us. The continuity differences between TWOK and TSFS may not have been as obvious (or important) to an audience three years later, than they are to us when we can watch the movies back-to-back at home.
Trek's not alone in this regard either, there's a whole bunch of movies where the differences get accentuated by our ability to scrutinize them so closely (one example I've cited before is Back To The Future II; there was much talk in contemporaneous reviews about how stunningly accurate the 1950s scenes are to what we saw in the first movie, and in many ways they
are, but the things that stand out as obviously different are much
more obvious to us thanks to being able to watch the movies back-to-back instead of four years apart).