I am not sure I agree this time. At least not from a fluff point of view. I see your point gameplay-wise though. I imagine the attacks being like an attacking Carnosaur in the PC game. He just munches into the group with his teeth and he will hurt several individuals with them. His claws are "raking" through the enemy lines. I don't see why that damage shouldn't spill over.
His claws makes sense, he has both the strength and reach to hit several (smaller) targets. So he can easily send a bunch of skinks flying. It already gets a bit questionable against larger units, say against kroxigors. But it's at least "passable". However his bite does not. It's a bite, it'l only hit whatever he can stuff in his mouth. And well, it might be a big dinosaur and possibly can swallow a human whole, but his jaws are nowhere near big enough to catch multiple targets. Not unless his targets are all neatly stacked up or Climbing into his mouth... It's not like he has tusks with which to sweep through them in his mouth. Also, in the pc game, his biting attacks don't hit multiple opponents. His sweeping attacks with his tail and neck do. Or whenever he just sorta jumps into a group of enemies. The actual bite doesn't. Anyways, the carnosaur might not be the best of examples that I Could've come up with, at last he has some sweeping attacks, but there's plenty were it doesn't make much sense for wounds to carry over... Anyways, it's just sort of a missed chance. Would have loved to have to make choices like "do I shoot this wounded monster with heavy artillery, and be certain it's dead, or use a lower caliber attack and save my big attacks for another target?" Or be able to overwhelm a powerfull monsterslaying hero with a horde of skinks in melee simply because the big bad axe of monsterslaying is only going to hit 1 skink at a time and there's a just too many skinks.
Your reference to the PC game has brought to my mind this: look at the proper Kroxigor's bite animation (around 0:47), I believe that is what GW had in mind when its game designers wrote the ability. It may be pointless to inflict all that pain (damage) to a simple human/Aelf, but the same strategy could be much more profitable against a minotaur, an Odor or something big! [P.S. I have read the last posts with a bit of haste, and I haven't put to much attention in them nor entirely understood, sorry... This means that if I said something outdated or not pertinent with what is being discussed now, simply ignore this post! And sorry, of course ]
pretty much yeah.. unfortunatly we have no distinction for damage on any other attacks. So for the sake of simplicity & consistency, and to Ensure that kroxigors dont just become useless cuz they're the only ones suffering from it, I'd say it targets the unit, not the model. Even if ultimatly it's a bit stupid and the clip makes much more sense. As said, kind of a missed oppertunity...
Basically, 40k. One of the few things where the King of massive destruction is more logical and balanced...