the only way i can explain this strange news, provided it's true, that 500 pg/ml was the cut-off level, there could be some unofficial interim threshold established for the time being or, more likely, that the ucla lab's detection method saw something but could not reliably estimate the concentration below 500 pg/ml.
the former in my view is highly unlikely whilst the latter is feasible - see contador's case infamous 'news' with '4 positives' when cologne lab seemed to face a similar situation.
to avoid getting to technical or speculative, i'll just leave it at that.