Aha! Good eye. So as far as I can tell, that's because Aristotle assigned air and water one way, and the Stoics assigned air and water the other way. Hellenistic astrology followed the Stoics, and geomancy followed Hellenistic astrology. But when astrology was revived in the West during the Renaissance, it kept the Stoic symbolism (in both astrology and geomancy!) but used the Aristotelian elemental assignments, thus making a big old mess of it. Modern astrology, of course, follows Renaissance astrology (and is, if anything, an even bigger mess).

This is a large part of the reason I took up studying Hellenistic astrology rather than Renaissance astrology or modern astrology: for me, it's very important that a system has very pure and consistent first principles, since I'm the kind of person who, if given bad theory, produces bad practice. (That's surely a water thing; I imagine airy people can do much better working with whatever they have to hand.)

(Also my veneration for Porphyry is very difficult to overstate, and he's one of our sources on Hellenistic astrology. :) )