For a Clueless Hockey Dad, you sure seem to know a lot that other people may not know.
Brown had a new coach last year so you can probably assume that there was a clash of coaching/player style with the two players not on the roster this year. My guess is that the Freshmen class comes from the previous coach as well as most new coaches do not get to bring in their own recruits until year three. So again, either the new coach let the previous commit go, or the previous commit decided to play somewhere else.
Dartmouth is also a relatively new(ish) coach so either Puddifant was not coach's recruit or there was another reason why she's not on the roster. Could be grades, could be something else.
There's no direct answer as to why classes come in big and finish small. For some players, they weren't happy with their ice time so they quit or transferred, maybe school or something in their life changed directions, etc. With the recruiting rules swinging back to coaches only being able to talk to older kids, hopefully this will help in attrition as players can make more informed decisions. The only unpredictable element is that the coach that recruited you in may not be the coach you play for.