Agreed it's not a valid comparison if you are not going to account for fertilizer used by the beans.  They weren't giving potash away last time I checked.  The biggest thing with beans being better in a "dry" year, especially on some of the lighter timber soils, is that if they can just stay alive it only takes one lucky late-season rain to make a respectable crop.  Corn that has already aborted a lot of kernels doesn't have nearly the ability to compensate and get that yield back.  Or, they can both just plain die and suck like 2011 here.