The DSM-V is about clinical diagnoses. Mental disorders are understood as something everyone experiences to varying degrees. You're considered to have something

The DSM-V is about clinical diagnoses. Mental disorders are understood as something everyone experiences to varying degrees. You're considered to have something clinically when it impairs your daily life, something psychiatrists have defined as being 3 σ away from the average number of symptoms someone would report. In this example struggling with things like math wouldn't qualify as a disability unless it impaired you in a statistically significant way