its just part of Darwin's theory of evolution.
All living things, (even us) will throw up a wide range of genetic differences or mutations if you like, where sex is concerned.
Human's are the easiest example to look at as we see this regularly, we are not all created as just males and females.
there are many people who are female in a male's body, males in a female's body and some who are physically and mentally half male and half female.
There are also a whole range of sexuality states between this and what we would consider the normal male/female sex.
Added to that, chickens will be subject to the same sexual orientation differences that other animals are, its quite common to see homosexual or bisexual activity within chickens.
I know a friend with a Buff Sussex cockerel who has NEVER looked at the hens but he just CANNOT resist a duck walking past!