The fact that the world was mundane, and levels limited to about 6th, 200 years before the first game was played. The magical world invaded, bringing with it demihumans, a FEW humanoids and monsters. The widely scattered entry sites are still infused with magic, and responsible for localized strange creatures.
That and the Royal Geograpic Society is trying to reclassify living creatures, abberations (solitary mutants), magical beasts (stable breeding populations), legacy monsters (those that arrived from another world in scattered locations 200 years earlier) and Outsiders - (including elementals)
Which allows rangers in the RGS to decide whose classifications are being used. (although in some cases subcatagories take presedence)