We’re looking at ADHD as either the “hyper” kid or the inattentive type. This article describes a third type of ADHD–the hyper-focusing child. They have pockets of passion that they spend 3 hours on, and look inattentive or overly-active when they aren’t intently doing one of those passions:
http://www.childmind.org/en/posts/articles/2013-9-23-hyperfocus-flip-side-adhd
One of my all-time favorite kids (and yes, he hyper-focuses and knows it) sent me the article via his Mom, who is a Harley teacher.