In either the first Librum Equitis or the second, Hellhound came up with some reasons as what it means when a PrC is 5 levels, or 3 or 10.
I believe 10 levels is seen as a natural progression of any of the core classes. 3 or 5 levels is seen as a specialist PrC and is solely focused on one aspect. However there is no reason why you can't have PrCs with a certain amount of levels. And also remember, in 3e originally a character could only ever get 20 levels. If the requirements for the PrC meant that the earliest anyone could attain the PrC were after 10th level, then what was the point of having a 10 level PrC?
Let's see if Hellhound answers about the Librum Equitis thingie or if someone has a copy and can find it in there.