Qabalah Theory Question

["metaphysics0000" <crispinp@...>]

Hello Everyone,
I hope this is not beyond the scope of BardonPraxis, but I have been
studying the Qabalah and an extension of IIH Theory, and have come
across the following paragraph in The Mystical Qabalah by Dion Fortune
that I can't quite understand:
----
24. The final system of classification which we must note is under the
presidency of the Three Mother Letters of the Hebrew alphabet: Aleph,
A; Mem, M; and Shin, Sh. These three, according to the Yetziratic
attribution of the Hebrew alphabet, are assigned to the three elements
of Air, Water, and Fire. Under the presidency of Aleph is the Airy
triad of Kether, in which is the Root of Air, reflecting downward
through Tiphareth, the Solar Fire, into Yesod, the Lunar radiance. In
Binah is the Root of Water (Marah, the Great Sea), reflected through
Chesed into Hod, under the presidency of Mem, the Mother of Water. In
Chokmah is the Root of Fire, reflected downwards through Geburah into
Netzach, under the presidency of Shin, the Mother of Fire.
----
My Question is: If The Pillar of Severity is Water, and Binah and Hod
correspond to Water, why would Geburah be Fire? And vice versa for the
other pillar. She casually mentions that water is reflected from
Binah, through Chesed into Hod, but doesn't explain why.
Thank you for any help!
Best Wishes,
M