java - YourKit is showing me that although my code makes use of 8 fixed threads, the JVM seems to be switching which 8 threads are actually being used. Why?

Question

I'm running YourKit over a simple program I've wrote that tries to solve the Traveling Salesman Problem by brute force, running several threads at once (in the current case, 8).

As the below picture shows, it seems that instead of having 8 fixed threads during the program's life-time, it seems to be using 8 threads for some time, then other 8 threads for some other time, and it seems to keep on switching each couple of minutes.

I was curious about this behavior. Has anyone any kind of extra info on why this happens?