Well rating scales don't have any absolute meaning, only that which is attributed to them either by the reviewer or the audience. On any aggregate review site the scale of the aggregate score will take on its own semantics, which is the aggregate of all the users' own semantics they use for their individual scores.
And for most people, it seems like the semantics of rating scales don't align with a normal distribution about the 50% mark, but something more like letter grades where 70% is decent/mediocre, while 50% is a failure (in the US).
IMO this makes at least as much sense as centering around 50%; if you only score 50% of the points on a test I don't think you're competent in a subject, and if your book only meets 50% of the criteria for a great book then it might be a bad book. But again, it's all arbitrary and only makes sense in the context of the particular reviewer or aggregate community.
And for most people, it seems like the semantics of rating scales don't align with a normal distribution about the 50% mark, but something more like letter grades where 70% is decent/mediocre, while 50% is a failure (in the US).
IMO this makes at least as much sense as centering around 50%; if you only score 50% of the points on a test I don't think you're competent in a subject, and if your book only meets 50% of the criteria for a great book then it might be a bad book. But again, it's all arbitrary and only makes sense in the context of the particular reviewer or aggregate community.