Originally posted by knighttemplar1960:

They may have altered things for chickens or it may be a bug. Game designers do like to reuse code where they can to save on programming time.

Maybe with chickens it's just 'lay an egg' every 10 minutes, instead of just a 'chance'. Unlike other animals, you still have to also wait several days total, for them to hatch and then mature, before you get another chicken out of out.