Thanks Windsurfboy. I now think I can see the difference. If you view all P and O cruises through this message board, then they do appear to be in order.
However if you use a search engine to search, for example, 'Britannia Cruise Critic reviews' (probably how most people read reviews), then the reviews appear in a random order. At the end of the day though, all reviews are very subjective, if interesting to read.