For some people, it is what one is able to do in the summer before and during senior year, rather than what will be done in the gap year, that drives in decision. The applicant has an additional year of grades which can bump the GPA a little bit, an additional year to schedule and take the MCAT, additional time for research to amount to something, additional time to take on roles as a tutor, campus leader, community leader.
In some cases, the decision to pursue medicine was made later than fall of freshman year so some additional time is needed to take the pre-reqs and be well prepared for the MCAT which shifts the earliest one can reasonably apply to the summer after college graduation rather than the summer before senior year.
If the application with just forecasted hours gets you the interview, what you've been doing since graduation is always a topic of conversation that an interviewer may wish to pursue and which can be very interesting for me, as an interviewer, to hear about.
I hope that this helps you to understand why a single gap year can be useful to some applicants.