Different relative gear levels will likely make a difference too.

The gear you have at 5.0, 5.1, 5.2 and 5.3, vs a given boss (even from the same point of an expansion... ie. 5.0 gear vs 5.0 boss and 5.3 gear vs 5.3 boss) could change the relative order of best boxed tank.
I don't see the "ease" of boxing a given tank, to change that much over an expansion.