User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-15400750-20130815061529/@comment-5020152-20130815194824

I personally think there's some kind of "land bridge" that allows the Deserted Island to be connected to the mainland.

As for Uragaan and others - it could be for a number of reasons. The monsters encountered in Free Hunt could be random wanderers that were booted out of their territory, or youngsters looking for food or a mate and have gotten lost. Some might even migrate there regularly for mating season or to seek additional dietary supplements.