Who knows. In the past year we've regularly gotten non consecutive boarding numbers even when checking in with our entire party on the same reservation at the same time. I'm not sure how it happens, but it appears to be standard at this point.
we had it happen yesterday too....a35 then a37 and a 38
the person who had a36 was probably checking in at the exact same time we were....we had three to check off, she only had one.....our first name beat her but then she was next in line to get a pass, then the other two names on our passes got theirs