When a recipe matches only a single system, the scheduler bumps its priority as a kind of heuristic to give more weight to recipes which need rare hardware. However when that happens it's not recorded in the job activity, so it's hard to see why your recipe set priority is magically changed to some different value than what you originally submitted.

If the scheduler changes the priority on a recipe set it should be recorded in job activity as a change made by user=None service=Scheduler showing the changed priority.

See also bug 979234.