everyones wrong...
...the reason for the difference in HP rating has more to do with just engine management difference, slight though it may be, b/w the auto and manual...not a big deal, but enough under WOT to make a difference.
It's true that technically they could claim whatever they want...however, they all agree amongst themselves to test under a set of conditions...whatever the current SAE XXXx method is now...and that probably now includes testing the same engine with different engine management schemes such as it'd be for auto/manual.
And yes...they don't claim there is no HP varience b/w engines or anything...many would say it's can be as much as 10-15hp b/w low and high side...
Also, many of the old "assumptions" about HP to the wheel losses are becoming less accurate as they now build for less friction, and to minimize rotating mass...while some old cars might show 20-30% loss at the wheels, newer ones may exhibit less than 10% in some cases