So there is a great post by /u/the_howling_cow which covers the general recruitment and allocation of the U.S. Army in WWII and I don't think I can improve on it.

But to answer your specific question, the only way a small town unit would come about is if they were from a pre-war National Guard unit. Even then, the process of "filling out" the army with draftees meant that a lot of men in the national guard divisions were transferred into another newly formed division to form it's cadre and help train the new draftees, and their places filled with other newly drafted men in a process that might be compared to cell division. Since most U.S. army units did not get shipped out until 1944, many of the guard divisions had been subject to this process repeatedly, and their local character was pretty watered down. Once the division got into combat, the pace of this process accelerated as local men who became casualties were replaced with nationwide draftees.

However, this wasn't always the case. The 2nd and 3rd battalions of the 168th infantry regiment of the National guard 34th division, based out of Iowa, was shipped overseas in early 1942, before this process could be completed. It was surrounded and surrendered at Kasserine pass in the U.S. Army's first serious campaign of the war in Tunisia in 1943. It suffered 2200 casualties and had 1400 prisoners taken, effectively annihilating it.

Source: Mansoor, Peter "The GI Offensive in Europe: The triumph of American Infantry Divsions, 1941-45"