Same thing happens over at Mini alot! I remember a press article in Autocar & Motor, who had a long term Cooper S on test (the newer turbo PSA block model) and I remember them going to a 'rolling road day out' with the Mini Club of GB. The standard car should put out 175BHP & 172lb ft. Their car was putting out 208BHP and 211ft lb.
Naturally they were very impressed, but in the article they made a notable overview that with that power why fork out more for a JCW?!?!? But then maybe JCW's run 230BHP and up - in real life
(Should have had my 2010 JCW 'Lucy' dyno'd)
Apparently (through the grapevine), some other manufacturers take a handful of output test results, chose the best and publish it globally. BMW seems to stand behind what it can 'definitely' provide.