1. There is a Qabalistic principle (see the Sepher Yetzirah) that Air is the reconciler between Fire and Water, and so neither Water nor Fire can be attributed to any Sephirah on the central pillar. They alternate sides, while Air takes the central balancing point betweem them.
2. The Elements of the Grades are assigned based on the Tetragrammaton: Yod (Fire), Heh (Water), Vav (Air), Heh Final (Earth). Since we work our way backwards up the Tree of Life, this results in the order of Earth (Zelator), Air (Theoricus), Water (Practicus), Fire (Philosophus).
3. The Flashing Colours of the lowest Triad in the Sephiroth reveal the Elements of the Grades. Netzach is Green, and its Flashing Colour is Red (the colour of Fire). Hod is Orange, and its Flashing Colour is Blue (the colour of Water). Yesod if Violet/Purple, and its Flashing Colour is Yellow (the colour of Air).
4. There is a mystery to discover regarding the strong relationship between Air and Water. Consider, for example, the watery symbol of Aquarius (an Air sign) and the airy symbol of the Eagle used for the higher form of Scorpio (a Water sign). Consider also that Air and Water share the same line in the Supreme Ritual of the Pentagram.
There is, of course, more to these attributions than the above, but this should be sufficient to show that the assignments are accurate as given.