From my experiences, it seems to come from complete and utter ignorance about what adhd actually is, and to a lesser extent the ignorance towards coping strategies and the effort it takes to implement them.
People find it surprising that I don't exhibit the symptoms they have incorrectly decided are the main issues, and exhibit symptoms/ behaviour that don't fit their stupid criteria, despite being classic adhd.
Plus the assumption we're all a homogeneous group, with the same personality, lifestyle, interests, skills, weaknesses etc.
My favourites are
'But you're so observant and notice every detail'
Yes, because I don't have any filter you fuckwit, I just happen to have a memory that can usually store most of that surplus info and a career where that's a blessing rather than a hindrance.
'But haven't you grown out of it?' Yes, because your cousins neighbours Aunt was dx as a child, didn't sleep for a year on Ritalin and now doesn't have adhd anymore. Obviously we all grow nt brains at 18.
'But when you're doing x a bomb wouldn't distract you'. Yes, hyper focus clearly wasn't mentioned in any of the armchair self dx experiences you've enlightened yourself with.
'But you're so laidback and calm when it's chaotic/ in a crisis' Yes dickhead, because that's incredibly common in adhd, and fairly self explanatory if you knew anything about what adhd actually is.
And my all time favourite, the fact I'm fairly well organised and don't lose my keys/ phone. Cos naturally they are the main criteria for armchair self dx, so anyone with a dx from childhood should exhibit them. Rather than the fact rigid routine and physical organisation, helped by a surplus of energy and little need for sleep, are my main coping strategies to balance what's going on in my head.