It happens quite a bit it just doesn't get reported so very often but it is not stange for this to happen. An other thing that you will see is one of the forks dying due to to little development time being spend on it and the left over core developers moving over to a different fork of the same root.

Another often seen situation is several opensource projects, be they forks of the same root or not, providing updates and bug fixes to another project whose code they use. The open source community is a very incestuous bunch in that respect, but they seem to like it and so far it works well for them.