I'm thinking that Kubo simply goofed. He needed Ichigo to one shot some dude in the first assault and he didn't want to use any of the actual characters he'd come up with so he tossed in Shaz for that sole purpose. Then he didn't consider how that would add up with the letters and here we are.
Oh, and for those who are wondering. Here's what the Stern Ritter alphabet looks like.
A – Ywach and Uryu Ishida
B – Jugram Haschwalth
C – Pernida Parnkgjas – “The Compulsory”
D – Askin Nakk Le Vaar – “The Death Dealing”
E – Bambietta Basterbine – ”The Explode”
F – Äs Nödt - "The Fear"
G – Liltotto Lamperd – “The Glutton”
H – Bazz-B – ”The Heat”
I – Cang Du – ”The Iron”
J – Quilge Opie – "The Jail"
K – BG9
L – Pepe Waccabrada – “The Love”
M – Gerard Valkyrie – “The Miracle”
N
O – Discroll Berci – "The Overkill"
P – Meninas McAllon – “The Power”
Q – Berenice Gabrielli – "The Question"
R – Jerome Guizbatt – "The Roar"
S – Mask De Masculine – “The Superstar”
T – Candice Catnipp – “The Thunderbolt”
U – NaNaNa Najahkoop
V – Gremmy Thoumeaux – “The Visionary”
W – Nianzol Weizol – “The Wind”
X – Lille Barro – “The X-axis”
Y – Loyd Loyd and Royd Loyd - "The Yourself"
Z – Giselle Gewelle – “The Zombie”
By process of elimination Gordon is N.