There is nothing random about it. If you look at the US Army Infantry Divisions that either currently exist, or ever existed, they are (mostly) sequentially numbered from the 1st Division up to about the 120th Division. Most of them are inactive and have been inactive since WW2, because you don’t need that many divisions when you are not actively at war.
Some of the Airborne Divisions started out as Infantry Divisions, and kept their numbers. The 101st Airborne started out as the 101st Infantry Division, and they kept their number when they became an Airborne Division.
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