This can only happen if your application task has the same prio as the idle task i.e. the lowest possible prio. Then the scheduler schedules idle task and your application task in a round robin/time sliced scheme. This is not really the idea of the idle task which should be the absolute lowest prio task and hence only run if there is really nothing else to do.
So better start at lowest/idle prio +1 with your application task priorities.