...missions. Some go on their mission straight out of high school and don't enroll until they return so they don't impact this, but a large number play a year before going and this hurts. In recent years Bronco has had some huge recruiting classes. If Bronco recruits 27 players and 3/4 of those go on missions but 1/3 go right out of HS that means potentially 15 or so kids who impact the score. This happens year after year.
Also, every non-member (or member) recruit who has Provo Culture shock and gets homesick and leaves the program affects it too (Fabuluje, Keoshian, Jerry Bruner, etc).