There is a difference between being selected as a preferred sub and being a sub on a district's or teacher's preference list. As a sub on a preference list, you may see jobs from certain schools or teachers before other subs. If you are selected as a preferred sub for a specific job assignment, you were selected by the person entering the absence and the preferred sub for that assignment, meaning, Aesop/SubFinder will try to give you the job first, but if you don't accept within a certain timeframe, the job becomes available to other subs.
A set of conditions set by your district determines how preferred subs are handled. Each district can customize these conditions, so we can't say exactly what they are. Also, even if a teacher selects a specific sub as a preferred sub, the Aesop or SubFinder system doesn't always immediately or exclusively offer the job to that sub. Each district can set up their system differently, but generally, if the absence for which the teacher is entering the request is soon (e.g. the same day or the next day), the window to offer the job to the preferred sub may be very short or already passed, causing Aesop or SubFinder to offer the job to more subs than just the preferred sub the teacher selected.
Also, a teacher's absence may be subject to administrative approval, so an absence entered by a teacher doesn't always immediately generate a job offer for a sub if the teacher's absence must be approved.
Learn more about SubFinder preference lists from our blog.