Yeah, pretty much this. I can speak for my institution (which filled this year actually) and I'll try to explain what I'm trying to say - we certainly don't start the interview/ranking process fantasizing about SOAP'ing people who fall through the cracks of subspecialties, we would absolutely prefer to fill with people who take the time and effort to express and interest in the field as well as our program, it's a total no-brainer there.
We have had no issues filling our categorical spots, and we often interview somewhere between 8 and 10 applicants per categorical spot on average (most programs do as well, when we asked around). This year we ranked about 3/4 of those, easily above 5 applicants per spot. But for advanced spots we simply don't get the same level or even near the same number of applicants on average. The last few years we have only interviewed between 3 and 4 applicants per advanced spot if even that many (we generally restrict our invites to competitive AMGs and stellar IMGs which knocks a large amount of the applicant pool off right away), and again since we filter out 1/4 with the interview process that leaves about 2 or 3 ranked advanced applicants per spot. That simply isn't enough to reliably fill the spots, we have found. So we are in the process of transitioning to mostly categorical or R spots.
Simply put, we would rather not fill and be forced to snag some people from SOAP for those spots rather than take undesirable candidates. But we certainly don't go through the interview and application process with this goal in mind (and the algorithm wouldn't easily allow you to plan that way unless you didn't rank anyone)! It's a total gamble and we could easily end up with someone who could jump ship and head into that subspecialty if offered (and we have had it happen). We could interview more for advanced, and we were more lenient with our cutoffs for that group but we can only safely go so low for step scores and clinical performance before it becomes a risk - especially if those scores haven't shown any sign of improvement. Plus we aren't going to rank interviewees who turn us off on interview day. We are largely restricted from taking all but the amazing IMGs because of some political pressure from the GME office I'd rather not go too much more into, it's an institution-wide policy.
Finally, applicants that come through not-so-secretly discuss the lack of ability to fill spots and it seems to be interpreted in a negative light.