Sex determination often causes difficulties in kittens: genitals at kids are hardly noticeable. And to be guided by existence or lack of "cambers" it is useless: the scrotum at little cats is often almost indiscernible, but newborn kitties under an anus can have swellings. Therefore, defining a sex of a kitten, it is necessary to be guided not by the size, and by a form and a relative positioning of genitals.
When people think of getting a kitten, they first of all solve they want a cat or a cat. Between behavior cat's of both sexes there are essential distinctions. Therefore it is so important to be able to define a sex of a kitten from the very first days of his life - to owners to attach kittens to the correct house, and who is going to get that a fluffy lump not to be trapped.