This is bad analysis. Disability is not like height, normally distributed in the population. There’s not a bell-curve of measles or lung cancer, and we have no particular reason to believe there’s a bell-curve of learning disabilities either. People who have dyslexia, for example, have qualitative differences from people who don’t. The incidence of dyslexia is whatever it is; there’s no mean or standard deviation of dyslexianess.