So the start of the screenshot is a process going to sleep for 15usecs (coming off the CPU) which means it's now in IDLE. At the start of IDLE it unblocks all sleeping processes, which is PID6, and moves that to ready (note that it only unblocks the sleeping processes that should be awake at the start of this time, so it won't do PID6). It then runs the next in ready queue as nothing else in the priorities is required.