Alterac Valley Bonus Honor

If you're referring to the daily Call to Arms, keep in mind that you gain a larger chunk of bonus honor for the first time in a given day that you complete it.

Your assumptions about honor are not quite correct. All the objective-based bonus honor is awarded to the entire faction - for destroying towers, defeating the general, etc. And no honor is awarded at all for doing damage or healing (on their own).

But honor awarded for honorable kills is different - it's split evenly among people within a certain range of the kill. (If the people were not all group members it would be split unevenly, but since everyone in the battleground is grouped this doesn't come into play.) This is fairly easy to confirm; if you stand near a heated battle you'll see small honor awards rolling in, and if you stay in the entrance tunnel you won't.

The range seems to be generous - this is why it used to be somewhat fashionable for people to farm honor by staying at the blacksmith in Arathi Basin. Not only would they get a defender's buff that increases honor, but by staying in the center of the map they would be within range of most kills that occur at the other nodes. (The range for receiving HK credit is not long enough to reach from the lumber mill to the gold mine, or the stables to the farm.)

And since HK awards are the only honor in battlegrounds that isn't awarded to everyone (except for the daily bonus), I'd say the answer to your question is probably that the people at the top of the honor list have been actively maximizing their HKs - by staying within range of the heaviest fighting, and by staying alive as much as possible. (For example, defending a bottleneck is a good way to do this.)

Also, it's worth noting that in AV it's quite common for people to come and go during a match, so at the end of any round there will always be a fair honor gap between people who were present at the start of the match and everyone else - even people who join only a minute or two late will often miss the bonus awarded for killing the faction captain.