Ill see if I can get some screenshots on here since the original ones are now broken links.

But think about it this way, when you start versus, your health bonus is at 200, with no medkits. And when you are all dead, its at 0. Notice that if nobody picks up a healthkit at the start, it will stay at 200 and remain that way until you start taking damage for any teammate.

Or, conversely, if you all have healthkits but lose 0 hp, you still gain 200 bonus points for not losing any hp.

And, you'll notice that if you have 3 teammates with healthkits and 1 teammate with no healthkits but 1 hp, you gain health bonus when healing him. But if he had a healthkit, your health bonus is still roughly 150+ because all 4 survivors have healthkits (or high health) and would end up green once the healthkit is "auto-applied" after the saferoom door is closed.

So it makes sense to heal people the most injured person without a healthkit at the very end of each round to increase the average health for the team thus increasing the health bonus (since healthkits have diminishing returns for coop purposes, it heals roughly 80% of the life that is missing).