Because the types given to some monsters in the Core make no damn sense.
For example, centaurs. They're monstrous humanoids in the MM, but lots of other human/animal hybrids (like lamia, sphinx and lammasu) are magical beasts. For consistency's sake, centaurs should probably be magical beasts as well.
Speaking of magical beasts, having ropers being magical beasts is nonsensical. They resemble no actual animal, have an alien physiognomy, and have plenty of tentacles. Clearly an aberration. (I'd also argue that umber hulks should go the other way, from aberration to magical beast or monstrous humanoid, but they're not in the SRD and therefore not pertinent).
Athachs, if they're going to be kept, should be giants. The only aberrant thing about them is the third arm. Rust monsters seem more like magical beasts. And derro don't seem very monstrous--bring them back into the humanoid fold.
Now, there's a few that are less "glaring oversight" and more matters of personal taste. Making some of the more bestial outsiders (hell hounds, hellcats, bebiliths, howlers) magical beasts with the extraplanar subtype rather than outsiders. Or making hydras and behirs dragons rather than magical beasts.
