That is something I have never been able to understand, everyone I know who keeps chickens either has cats or their neighbours have cats. Why would the cats ignore the chickens?
In general, to a cat, "runs away" == "prey" and "comes right up to me" == "equal and/or potential threat"
Partly aggression - chickens are/can be nasty and will attack other animals with sharp beaks. Combine this with the fact that they're flock animals, one cat isn't going up against multiple flying, aggressive, pointy ended foes.