Two reasons. Firstly, commonality with the MkIV formations, which have nine passenger vehicles and used to be A-B-C-D-E-G-H-J-M or A-B-C-D-E-H-J-K-M depending on the number of FOs. With GNER, MkIVs used to have smoking facilities in coach A, so that wasn't included in HSTs. When smoking was banned, coach A disappeared (becoming the luggage compartment in the TGS). The other reason is to avoid having a Coach I, in case people think it's Coach 1. Everyone else stuck with 2+8, so the Coach I issue never came up. I believe there was a Coach X when NER (as was) borrowed some WR coaches because WR had a lot of traction motor failures. WR also had 50s on MkIII formations for a while, not sure how they handled the incompatible ETS,