I don't care if I'm paying towards some little kid's chips in a bill split but other people seem very mathematical about it. Financial pettiness always irritates me.
I think some people just don't want to pay for things they didn't have. A few of my own recent examples:
I went out with a few people and they ordered an expensive bottle of wine, which I did not have any of. They expected to split the bill evenly. Sorry, I'm not paying for you to drink fancy wine.
I went out with a couple friends and we got an appetizer. I had one little bit of it, but the waitress decided to split it three ways between us when splitting the bill. I didn't really say anything about it, since it wasn't that big of a deal. I'd never do a breakdown about how much I ate vs. the others.
I went out with a friend, we both had comparable meals, except I ordered a couple slices of pie to go. I'm not going to expect him to pay for one of those slices which he won't even have, so I sat there and split it up.