I think you got it pretty much complete there, I suppose all I can add is why I wouldn't upvote, and that's where I think the author got a good reward already. So if someone has a $100 in upvotes, I'm not going to add to that, I'll give upvotes to some other less rewarded posts.
I'm not sure if I agree on that. It's definitely an interesting way of looking at rewards. For me I think a case like that would depend on if I know the person, or if just something let's say, a community post. If I'm a fan of someone, it doesn't matter if they have 1000dollar rewards on it already, I will gladly add mine to the stack too.