I also found, as did Melev, that surface water movement had a significant effect on par. With my powerheads off, PAR was much higher. However, since we don't run our tank without water movement (obviously), we only shared the readings based on our powerheads set to their normal flow.
edit: I just realized I am posting on my wife's account. Darn, I wanted to be Smart Brother and now she gets all the credit! lol