More likely, a college has assigned each admit an individual yield likelihood based on student characteristics (e.g. stronger students are less likely to matriculate due to having more other attractive admissions or scholarships), which may be influenced by the school by offering scholarships.
A college may keep a running count of the expected frosh class size and update it as admits matriculate or decline (i.e. if a given admit was estimated for a 10% chance of matriculating, counted as 0.1 student, then if they matriculate, they are now counted as 1 student, but if they decline, they are now counted as 0 student). If the running count falls too far below the target class size, the college may start using the waitlist early.