Yes, @foosondaughter, I have seen students make that case. But, the fact is that you simply cannot have a 4.2 UW on a 4.0 scale, b/c definitionally the top mark can’t be more than a 4.0. If the school treats A+ grades as a 4.3, then either the scale is 4.3, not 4.0 (and a 4.2/4.3 = a 3.9/4.0) OR the 4.3 is a ‘weighted’ grade, and the 4.2 is W, not UW. Agreed that a 3.9 or 4.0 is more in line with the class rank 
on a side note, OP, that is surprisingly sloppy language for a college counselor at a ‘fairly prestigious’ boarding school to be using.
‘matches’ = fits your stats, statistically you should get in, but you aren’t certain to get in (usually b/c too many qualified applicants overall)
‘safeties’ = you can be confident of getting in
fwiw, I have seen (first hand) students ED to a solid “match” school and be deferred and/or denied. The greater the imbalance between # of applicants & # of spaces, the more likely that is to happen, even for applicants whose stats are in the upper end of the accepted student range.