Mensa requires a score of 148 for these tests. The score of 148 represents exactly the same 98th percentile. An IQ score means nothing without the name of the test by which it was determined. I read of a woman who said that her son had an IQ of 178. Actually, he had taken the old Cattell IIIB, and that 178 IQ is equivalent to 152 IQ on the Stanford-Binet.
Standard deviation is a mathematically determined figure to account for variances from the average.
Furthermore, a true genius may not score particularly well on a standard group IQ test. We know a Nobel Prize winner who never scored at Mensa level on a school IQ test - he was too busy seeing all the alternate possibilities for each answer. At the present time, all IQ scores are read off of tables. Now there are almost no tests in use that will give extraordinary high IQs except those with very large standard deviations. And really, those who are what we may call a genius don't need a score to prove it.