IMHO, It is possible that the tests are a little more natural to those with a PPL than those without one and therefore those holding a PPL may perform better IN SOME STAGES.
I too, however, was accepted first time with virtually no prior aviation experience so it's not un-achievable. What I will say about the RBI test is that it is (probably) the most complex of the lot of the tests and it is therefore imperative that you read, re-read and re-re-read the instructions before you attempt it. When you've done that, read them again. It is the only segment where you are allowed as long as you like on the instructions for a reason.
But to answer the original question, no. I don't think it's biased in any shape or form...if you look at the often thrown accusation that OAA try and milk you for as much as possible, it would be the other way around as those with a PPL are more likely to pass flying first time and not need the additional training...