Was that in commuting conditions, or racing conditions? One research paper I saw supposedly demonstrating the benefits of clipless had pro cyclists exerting maximum effort in a lab with and without attachment. Of course they did better with the clipless since they weren't used to platforms and having to keep their feet on the pedals at max cadence instead of relying on the cleat to assist. Had they had a chance to practise with platforms for a while and cycle at submaximum effort, there might have been no difference.