This is a rough breakdown of how US divisions were numbered, at least in WWII.
1-25 were Regular Army divisions
26-59 were National Guard divisions
60-110 were National divisions
The national guard divisions are normally linked with 1-3 specific states and, at least until they hit combat, had a strong contingent for troops from the state. The national divisions were filled with draftees from the very beginning.
In the post war era, the 82nd and 101st were converted into Regular Army in honor of their combat record;wereas the 11th, 13th and 17th Airborne Divisions were deactivated and remained that way except for short periods, i.e. in the Vietnam War, the 1st Airmobile Cavalry Division had been the 11th Airborne Division (Airmobile) during its original training.
As far as the brigade/regimental numbering....there is a rumor that the Pentagon writes 1-1,000 on slips of paper and then draws numbers at the Christmas Party...
