The term "normal" has to refer to a range - not a point - exactly for the reasons you gave. Then it does make sense to make a statement like 90% of some population is normal.
Both in mathematics as well as in colloquial language a term's definition must be such that it is useful. The term "normal" is grammatically an attribute (something is normal or is not normal). If "normal" is defined such that no object can be called "normal" - then the the term "normal" is useless.
This way of thinking about language is not just logical and economical but also conforming with a friendly approach on communication. Of course, this is something unattractive to many people who celebrate their incredible and superior intelligence for the purpose of grandstanding. And I fear this attitude is relatively more normal on HN.
