By cross-ranking mean being awarded rank in one art because you have rank in another. Like if you have a BB in Kenpo and someone gives you a BB in TKD. Doesn't necessarily have to be high ranks, or ranks being exchanged, but that would fall under cross-ranking, at least the way I see it. This is bad because you now have a BB in a style you have no experience or skill in.
As to the contradiction, cross-ranking is, in my opinion, absolutely contradictory to the intent of rank. However, honorary rank, being that it is given as a recognition of some sort, and does not in any way signify actual rank, isn't a real contradiction to the intent of rank. Honorary, by definition, means conferred in recognition of achievement without the usual prerequisites. I hope I'm making some sense. Basically, cross-rank would be actual rank being awarded, honorary rank is not actual rank, it's "honorary". I wish I could come up with another word in place of honorary to better make my point.