If you aren't taking into account the length of the word, then it looks like the optimal choice of 5 consonants and 1 vowel. Look at his section titled "First refinement" the most common letters were (in order) "ESIARNTOL" Eliminate all but the first vowel and you get ESRNTL, the same as WoF.
Given your perception can generally fill in the vowels - picking any early on would seem non-optimal.
I think it's more a goal of eliminating uncertainty about which letter occupies a slot.
For example, if I have guess T and E and see _ _ E as the word I know it's not the word "The".