I have a theory, although it's pure speculation:
If Harvard interviews around 1000 applicants total, then each subcommittee persumably reviews 250-300 applicants post-interview (assuming that not all of the interviewees were divided up perfectly evenly into the 4 subcommittees).
From the interviewees, I am speculating that 250-300 will move on to the final committee. This is based on the size of the final committee being slightly larger but similiar to that of the subcommittees and the fact that the final committee probably does not have that much time to re-review 500 or so apps. Therefore, I think perhaps the main admissions committee gets a similiar work-load as that of the subcommittees. Then, their task would be to award 180-200 admissions from the 250-300 pool and wait-list everyone else who made it past the subcommittees. (Of course, there might be a rejection here or there if the main committee finds a particular reason to do so...)
just a thought 🙄