It’s a legacy from the rapid expansion of the armed forces in WWI and again in WWII. Originally, units were numbered sequentially as they were created with numbers reserved in anticipation of a certain number of units. If you look at the divisions that were active in WWII, they are all pretty much in sequential order.
After WWII, many of the units were no longer necessary, so they were stood down but not in sequential order. Some units were kept because they had specialized roles that needed to be filled, and others were kept because they had attained a legendary elite status during the war.
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