It doesn’t just have to be severe disability. I’m pretty sure I’ve got what is now called NVLD (because my son is diagnosed with it and I share most of his symptoms). It doesn’t cause general intellectual issues, in fact I scored above 130 on the WISC IQ test as a kid. But one of the things it does cause is poor working memory. Trying to keep numbers in my head in order to calculate things is pretty much impossible. Give me calculator and I have no issue with the higher math concepts. But leave me on my own and I’ll give you the wrong answer due to bad calculation every time.
Anyway, this all led to me doing very poorly in math at school, and it’s entirely because my brain just isn’t good at it. Luckily these days the educators are smarter and my son has always been allowed a calculator for math class.