I think this is a symptom that has a tricky differential diagnosis. My DS does it, but I don't think he literally means it - I think if someone else said he was rubbish at maths, he wouldn't just agree! In his case, I think he says it sometimes as an expression of a feeling of dissatisfaction that he doesn't know how to put words to (if I suspect this, I say "don't make statements you know to be false - if you need a cuddle or something, say that") and sometimes in frustration at having made a mistake (I say "no negative self-talk"!)
