Their motivation is that they interview a lot of terrific kids, not all of whom they can admit, but whom they want to encourage to complete their applications. At no point prior to the admission deadline has any AO/AD seen the entire applicant pool, but each has seen many wonderful, admissible candidates who would fit into their communities, and they are happy to encourage and convey what wonderful kids they are. Nothing wrong with that. Again, stay here long enough, read through enough M10 periods, and you will see how common these comments are and how non-predictive they are. You may also get cards and handwritten notes from AOs, and those are not reliable hints of decision outcomes either. The sense you make of it is just what is conveyed – your applicant is a delight and had a great interview/conversation with that AO/AD, as did hundreds of others. Read nothing into it.
For a school with a 10% admit rate (your example), almost every admit would be a positive surprise and no denial would be a shocker, again due to those odds of admission. For the schools with the lowest admission rates, there are no guaranteed acceptances for anyone outside the pool of those very few who are accepted for specific institutional reasons/relationships–and those applicants/families already know who they are. If you are posting here, you are not in this pool.
Where a lot of head scratching occurs here is with applicants who appear to check every box for a particular school yet are denied or waitlisted. The main reason: Too few seats for too many great applicants and no way to know, for any school, what type of class that school is trying to build/round out that year and who any given applicant’s competition is. That’s pretty much the story of every M10. Of course, there are always examples every year, too, of students/parents who are surprised that perfect grades and test scores did not produce the results there were looking for, but that shouldn’t come as a surprise because these schools are not trying to fill their seats with scores but with students who will fit in and contribute to their communities in some special way. The grades/test scores are used only to ensure that the desired applicants have a high probability of academic success in what are very rigorous programs.
On the college side, I would say there are very few surprises as the BS college admissions process is highly curated for success. By the time a BS student is applying to college, that student is well-known to teachers and counselors and the college list is carefully crafted for great results. There are many threads in this forum describing how unsurprising the BS college process is and why.