Its just a bit like kids, they can eat a big dinner and be too full to finish it but offer them sweets and they are all starving suddenly lol
Chickens love treats and they don't want another chicken to get something instead of them so they all fight over the treats thus looking like they are starving when they aren't really.
However If you are at all concerned they could really be hungry just feel their crops occasionally during the day, rub their chest and you should feel a big lump on one side which is a sign of a full crop.