Probably to keep the same "meaning" for a seat between aircraft with different seat layouts.
Say they have a 3-5-3 aircraft and also a 2-3-2 one.
So the seats on the 3-5-3 get labelled:
ABC_DEFGH_JKL
Then you might pick
AC_DEH_JL
for the 2-3-2 configured a/c.
That way the booking people know that seat C is an aisle seat, regardless of the aircraft.