From what I understand, they tend to stay true to the “order of the list” for the most part, but if there is a box that would fit better into the flow of the most current releases, then something else might be decommissioned so the new box will fit in better.
Netmon being decomm’d makes perfect sense to me. User has been owned by 100% of the active community and the box is getting pretty old now anyways. On top of that, there have been some decently difficult box released recently, which means replacing another Hard/Insane box wouldn’t make much sense.
I’m no expert on all of this stuff, but this has just been my observation in the ~400+ days I’ve been around on the site.
Happy hacking! 