>>11067201IQ tests are normalised to 100 with a standard deviation of 15, using a reference population. People are then compared to the reference population. It's arbitrary, but I guess having an IQ of 90 sounds better than having an IQ of negative 10.
IQ tests don't go much beyond three s.d.'s each way because they're very small populations and hard to study and it's not very relevent for the use of most IQ tests. If you're +3 s.d. you distinguish yourself in other ways, not by doing a test. If you're -3 s.d.'s people are going to be more interested in your actual capabilities (can you dress yourself? cook basic food? safe to walk to the shops?). It's also very hard to write questions for people who are functionally illiterate.
