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" Power measurements on dynamometers are not approved by Porsche."
and on the next page it says "...unless it on Porsche Dyno at a Porsche Dealer."
It may be a case of they got tired of owners coming in and complaining their dyno didn't indicate the book listed BHP. So they decided to tell everyone, we don't approve non Porsche dynos, so don't come complaining to us.