It can be both overdiagnosed and underdiagnosed! Both these things can be true at once:

• Some people who would benefit from ADHD treatment do not receive it.
• Some people receive ADHD treatment who would be better off without it.

One contributing factor is that people are treated differently in the contexts where ADHD is likely to come up (e.g. in schools) due to things like race, gender, and family income.

A black boy and a white girl having the same "brain stuff" going on are likely to be treated differently by teachers and parents — not only due to overt sexism and racism, not only hormonal and developmental differences, but also different cultural experiences with medicine (and medical mistreatment), different parental fears, etc.