In Unity ECS systems update in kind of a hierarchy - the player loop consists of 3 main sections initialization, simulation, and presentation. What you are seeing there is the plumbing of how the simulation system group works. By default (if you don't specify anything) Unity will schedule new systems into the simulation system group. If you follow the hierarchy down to the bottom, you see that the lowest entries on the profiler are your jobs that you have scheduled. Then in the next sections down you see your jobs also running on multiple threads. The simulation systems need to wait for your jobs to complete, so it it not that the simulation system groups are taking up so much time, but it looks like your jobs are taking up a bulk of that time (which is to be expected).
The reason for the gap is that there is likely another job that got scheduled in between your two jobs, you'll be able to see this if you scroll down the profiler a bit.
Hope that helps!