The reasons given make sense from a technical standpoint, as the TTAB's response to Honda's appeal states that "we find that the likely impression here is that ACURA is the trademark for a line (or "make") of cars, while RL is a trademark for a particular "model" of that line."
Well, yeah. Acura is the make and RL is the model -- we're in agreement on that one. The differing viewpoints stem from the fact that Honda was unwilling or unable to provide proper evidence to the board that the words "Acura" and "RL" appear close enough together on the car, its manuals or its marketing materials to warrant its own trademark. Nitpick much? Thanks for the tip, Kevin!
[Source: TTAB (PDF link) via The TTABlog]