From what I have been reading about colleges in general it is a tactic to boost college rankings and protect yield.
Also, by deferring more students to regular decision makes the process more efficient for admissions. By filtering to RD based on stats many students who are likely to attend another school will likely move ahead with enrolling in the other school to take care of housing and beginning planning. They withdraw their deferred applications. Then admissions has a much smaller pool of candidates who are more likely to attend.
It of course varies by year but over the past two cycles we have met our student enrollment goals of a little over 2k new students accepting their offer before looking at waitlist students so to be honest it is very rare to get off waitlist and growing increasingly more difficult but of course it does happen.
Frost handles for the most part its own admission process so yes it is slightly different, the comments I made above apply to the other academic colleges. But even then it is still easy to switch majors for the most part.