[url=http://halo.wikia.com/wiki/UNSC_Army_Airborne]82nd is an inferior Army Airborne division, and not an ODST-certified unit (Typ. Shock or Force Recon Battalion).[/url]

The other key difference is the Army Airborne uses Airfoil carpaces (jetpacks) and resupply-cans, and not SOEIVs. Although technically Pella [i]should[/I] be ODST-qualified Force-Recon, I can likely think of a reason why he isn't.
-In short, Army-AA is in-planet rapid-deployment (from Pelicans and such), ODST is exo-planetary.

[i]I figure 46th Shock Trooper Division is up for grabs.[/I]