Don't read too much into this. It is only a suggested human interpretation of a particular statistic. What is an appropriate facility index for the questions in a given quiz will depend on the educational context. However, the values in that table are a good rule of thumb. However, they are only a rough guide, so the exact numbers don't really matter. If the slightly sloppy presentation offends you, you can use the fact that the docs are a wiki an correct it yourself.
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