Actually, it is somewhat routine. It is usually due to a combination of the stress and trauma of leaving the cattery, the travel, and the immediate adaptation to a new environment, new food and new owners. As breeders, we get calls on that all the time. There are pure-bred cats available which are not kittens, but are older. Don't immediately reject the idea of buying a cat, instead of a kitten.
Cats are like people - they are all different.
There are any number of reasons. Most commonly, this cat has been neutered, or will be sold only if you agree to neuter it. Don't reject this out of hand. These cats are often ones which can get used to a new environment very quickly, and which can make excellent companions, particularly if you are not the kind of person who can take the initial frenzy of kittens in your life. As animal owners, we think that it is useful, if for no other reason than it protects us, from ever having to worry that we might hold off on critical medical treatment for financial reasons.