ADHD person here. When I was diagnosed in high school, I honestly didn't believe it, but I have come to accept that it really is a difference between myself and normal people. Normal, small, repetitive tasks are extremely hard to remember and keep track of. I need to write tasks down or perform them immediately, or I will commit and forget. However, when a novel and interesting task comes along, I can get lost in it for hours, forgetting to eat or sleep or even spend time with my family. "Interesting" is subjective, but I'm sure you could design a study that could begin to identify people.
One problem is that there's a pop culture understanding of ADHD and there's the medical version. Because ADHD is poorly understood and is diagnosed by ticking boxes, I get the feeling that some people just say what they need to say to get the diagnosis/meds/whatever. (I am unmedicated and have been my entire adult life. I felt a bit like a zombie on Adderall, and my heart was constantly racing.)
On the other hand, I do think there's probably a better, deeper explanation that has eluded the professionals. I suspect that ADHD has something to do with a lack of free play for young children. When kids are able to free play, they invent their own games and set their own boundaries and learn to regulate themselves; this does not happen when they are under adults' direction. As a consequence, we see that the US has extremely high ADHD rates in children, whereas in other countries, especially ones where early education consists almost entirely of free play, especially outdoors, is encouraged, the rates are quite low.[0] Further, within the US, we see that children born in August seem to have a higher chance of an ADHD diagnosis than those born in September.[1]
I suspect that the disorder is "real," but with a very strong "nurture" component. In the US over the past 60 years or so, children's lives have become increasingly directed by adults, from time spent in school to time spent in sports or clubs organized by adults to time spent at home doing homework. At the same time, Americans have become increasingly hostile to the idea of children's free play. "Where is that kid's adult?" is something I hear from other parents when I take my kids to the playground.
If I could summarize my own experience, I would say I have trouble self-regulating along the spectrum of being bored or interested in a topic. If I'm bored, or think I will be, I have almost an anxious response; if I'm interested, I may get lost in the task or topic till a phone call from my wife breaks me out of it. For what it's worth, when I'm healthy, I can rarely sleep more than 6 1/2 hours per night.
[0]https://academic.oup.com/shm/article/30/4/767/2919401
[1]https://www.pbs.org/newshour/health/could-being-born-in-augu...