Do understand your point 15thaugust ... particularly as explained in your 3rd and 4th posts.
However, it really does not sound as if you definitely know, for sure, what circumstances brought about the situation regarding this particular party.
For example, I could imagine this situation arising if the parents had told their child he could have a themed party ... so he picks a football themed party. The parents then book and pay for, say, ten children and tell their DS he may invite 9 friends. It seems to me perfectly possible that the child (with his football themed party in mind) might pick the 9 friends he's closest to from, say, an after-school football group, rather that the 9 friends he's closest to from school.
Is that possible? That it was simply down to the child picking a strictly limited number of children, with the theme tying in with, for example, an out-of-school activity group uppermost in his mind?
Although desirable, I think it's probably not always possible for parents to guide their children into picking a fair mix ... especially when numbers are limited.