Why? Maybe I simply only review things I like. Why would that devalue my reviews?
This. One reason really low and really high reviews are much more common than they ought to be is that people only bother voicing an opinion if they feel strongly (positive or negative) about something. Another is that once they do, they'll tend to exaggerate their evaluation to really drive home how they feel.
My suspicion is that the only stable scale is a simple "really liked it/really disliked it" up/down system. Then somehow weigh that according to the proportion of customers or buyers that actually bother to review. That depends of having a decently good estimate of that proportion though. The likes of Amazon have that for their products; for restaurants it'd be hard to impossible,