I think part of it has to do with cultural upbringings, too. Really, every region has it's own cultural values and perspectives, and I think a bit of that translates into the game.
As an example, when I lived in Toronto (I was pretty young, but I saw a lot of my dad gaming with groups) miniatures play was a big thing. The three different GMs my dad gamed with all used Minis... it may have been because they were in a military group, so that the tactical side of the game would have been appreciated more, or it might have been the fact that housing in this area was paid for by the military, so some of the gamers had more of a disposable income to spend on minis.
Whatever the case, minis were pretty popular.
Now, over here, I see gamers use minis, but not on the same scale that I saw in Toronto. We use them in every fight for my group, but we use a lot of "penny-monsters" because we just don't have the time, money, or inclination to buy 500 orcs.
That's just a small example, but I think if you look at surrounding situations, you can start to see how certain gaming trends emerge, even in completely isolated groups.
Look at all the groups playing over in Iraq. How many of them have contact with each other? Maybe a few, but I bet there are a lot of groups who only play amongst themselves, and have no contact with other Iraq gamers.
What conditions would be forced upon these groups?
1) Limited access to gaming materials (whatever you can bring with you, and I bet it's not a whole lot)
2) an "in and out" rotation of players (due to players moving within the unit, for example)
3) Irregular playing schedule
4) Cramped gaming conditions.
BTW, I'm guessing at conditions, here... I'm no expert on RPGs in Iraq, or anything like that.
But, using those four conditions, we can assume that many of the games will adopt certain characteristics:
1) Due to a lack of gamebooks (unless the players have pirated PDFs, which is another story), the games might wind up having a lot of home-brewed stuff available.
2) Lack of minis could result in a less tactical feel, or a more "role-playing" environment (although I'm not one of those people who think that minis always get in the play of RP).
3) Use of game rules that allow for odd-mixed groups (Gestalt, maybe).
4) Shorter adventures that could almost be considered one-shots that are loosely connected.
Not to mention the fact that, given a shared military background, there might just be a general preference for certain character classes (I've always found that the military folk I know have an unusual fondness for the ranger and paladin classes).
Anyways, the point I'm getting at here is if you were to take 1,000 gamers from a military background in a war zone, and compared them to 1,000 gamers from a university campus, you would notice that each group as a whole adopts certain play styles, even if they are completely independant of each other, due to the prevailing conditions around them.