That seems to be what happens, yes, although I realize a few tests aren't real proof. For what it's worth then: I did a few tests in the Cathedral of Darkness (Icecrown zone), because there are many mobs in there that respawn fast. Even when pulling lots of zealots from both sides, they run to me and stay in front of me, despite the fact there isn't really room for all of them to spread out (sometimes I pulled more than 25 zealots at once). They all spread around as nicely as possible (even if the results looks 'very crowded') in a 180 degrees cone (half a circle).
I got the same behavior in Stockades and Zul'Farrak when pulling like half the instance at once; though in Zul'Farrak casters stayed at a distance of course, while melee filled half the circle in front of me
But like I said: it's not a proof, but a good indication