The short answer: Teacher turnover.
The school board and ABSS central office are concerned about a lack of stability at particular schools and the cycle that perpetuates on a campus.
Teachers with provisional -- temporary -- licenses are either in classrooms on an emergency basis or as lateral entry -- teachers with degrees in subjects outside of education working toward their education certificate. Those positions have higher turnover rates (15.8%) than teachers as a whole (around 8.5 percent).
In ABSS, these teachers are concentrated at certain campuses -- which happen to be lower-performing academically.
The point isn't that these aren't good or qualified teachers. They may be excellent and enthusiastic, but they lack classroom experience and are more likely to leave teaching for a different job within several years.