Because the speed you gain TP compared to your delay is not completely linear.
http://images3.wikia.nocookie.net/ff.../3d/Tpgain.jpg
For the optimal haste situation for WAR's we're talking 23% haste in gear, 20% in BRD, and 15% in spell? 58% reduction then.
.42 x 409 = 171.78 (Ridill)
.42 x 504 = 211.68 (Gaxe)
As you can see, haste gets the Ridill just past the very sharp spike drop, putting it better off, relatively, to where it was before. Gaxe turns out to be roughly where it was before.
For only one march we're talking 49% reduction
.51 x 409 = 208.59
.51 x 504 = 257.04
Here again Ridill turns out better as 257 places at a decreasing part of the slope.
I believe it's the same sort of effect in Dual Wield (DW naturally favors weapons with less combined delay) which makes Koggelmander/Perdu so attractive to BLUs -- they can get into a "sweet spot" for delay that optimizes how reduction trades off with TP.