I think you are reading too much into it.
First, there are mandatory rotations. Most of the time these are noted in the job posting, but not always.
Second, there are non-mandatory rotations. To best serve the client's interests, it only makes sense to send in different shoppers if you can just as easily do so.
Third, there is keeping your shopper base active, involved and growing. With the relatively short 'lifespan' of most shoppers, there is a need to frequently try out new shoppers if at all humanly possible. You can certainly fall back and give the shop to one of your 'regulars' if the new shopper fails to perform for whatever reason as long as you keep your deadlines tight on the front end of your own scheduling 'window'.
So not getting a job that I did last month and the month before is not something that would get me concerned and taking a breather is not necessarily going to cause my devoted scheduler to love me any less.