...NCAA tracks graduation success over 6 yrs and the APR is calculated both annually on a 4 yr rolling schedule to get an overall average.

The APR is calculated as follows:

Each student-athlete receiving athletically related financial aid earns one point for staying in school and one point for being academically eligible.
A team’s total points are divided by points possible and then multiplied by 1,000 to equal the team’s Academic Progress Rate.
In addition to a team’s current-year APR, its rolling four-year APR is also used to determine accountability.

So if BYU has 85 players on scholarship and 65 are academically eligible at the end of a term and retained for the next semester BYU gets a point for each on being academically eligible and a point for staying in school (2 of 2 possible points x 65 players=130 pts out of a possible 130 pts)

If 15 players leave on missions for 2+ yrs BYU loses a point for each one (lets assume for the discussion that they're all eligible. BYU gets 1 out of 2 possible pts for each player, 15 x 1=15 out of a possible 30 pts).

If 3 players are retained for the next semester but academically ineligible that's again 1 pt each out of a possible 2 (3x1= 3 out of 6 possible pts).

If 2 players are academically ineligible and they drop out of school BYU gets zero points out of a possible 4.

This totals as follows - BYU gets 148 pts out of a possible 170 pts for a pct of .870 which is well below the minimum limit for one year. Fortunately they avg this with your rolling 4 yr score to help offset one year anamolies.

The mission deal is a huge factor because it's never a one year anamoly, players leave on missions every year. In my example even if only 5 kids leave on missions the rate is .929 which is below the rate needed to compete in NCAA tournaments or championship events.

IF a BYU kid comes in and plays a year then leaves on a 2 yr mission and then needs a year (redshirt) to gain back his weight and fitness and then plays 3 more years he could be at BYU on scholarship for 7 yrs. Hopefully he graduates during that time.