Rating, as a basic type, is an integer which is shown as stars (even half stars, added some time ago although not to the core column). And this is intended as the user's rating, not an average rating, that it is your request.
For an average rating or the result of a calculation, you can create a custom column "Number with decimals".
BTW, don't think in terms of "calibre gets", calibre gets nothing about rates, doesn't make any calculation, nada. It offers an special type and a column of that type if you want to fill it, that's all.
And all the data in calibre come from 3 ways:
- Metadata read when a book is imported in the library
- Metadata included through a metadata plugin
- Manual work from the user
There's a 4th kind of data, that it's an special kind of manual work: calculated column. The user has had to design them before.
So, in calibre you can define as many column for data as you like but... they have to be completed, don't forget it.