the problem with that Bet is
A. rBST has been Common for so long that Mom & Pop Dairy Farms had access in the 90s, 'Impossible'/'Beyond' are new kids on the block they don't yet have the exposure so of course the 20 year old product will do more 'X' than the 1 year old one...
B. Having been 'normal' for so long, surely some one would have noticed a distinct 'Moob epidemic' within the general Obesity issue of the last 30 Years