I've noticed that damage seems to "congregate" on one or two units in a stack over the course of a battle, but that might be because of reduced ability to defend, or other factors. Frequently, one unit drops to 0 Org, while the others are only down to half or so. There's some apparent tendency to attack the same target (not sure why), but each "round" seems to have at least a chance to attack a different target instead, so the "weak" unit's ORG and STR may drop for a while, then damage shifts to another target for a round or two, and then shifts back.
Indeed, one division gets the "lions share" of the shooting in any given fight.
I believe that is to represent that, in actual combat, the divisional leaders are trying to concentrate fire on specific targets.
Occasionally, the "main target" will change mid-fight, but the chance is low. 99.9% of the time, whoever takes the first bullet, will take 75% of the bullets, until they retreat or win.
Which doesn't mean that one unit will ALWAYS go down fast; although one unit is taking the brunt of the damage, it's still all units' stats that are getting throw into the calculation before the damage is applied, so more divisions will still reduce the damage to the main target, just not by "taking the bullets" for them.
It is a mechanic that can be somewhat abused if you want (if you have a real good combat reinforce chance, you can send in a super-resilient unit to fight first, then have other supporting units join in the hour after).
But it does make some logical sense that enemy commanders will want to knock out any given asset as fast as possible rather than dividing all their fire.