Which is exactly why diagnosis needs to be done by a professional.
ADHD has symptoms which overlap with a lot of conditions.
WRT the spectrum discussion below, it's my understanding that ASC is considered a spectrum condition as in, you're either autistic or not autistic and with autism comes a potential spectrum of different symptoms, characteristics or impairments.
ADHD is a bit different in that there is a bell curve of everyone's executive functioning level and most people will be in the middle, but some super-organised people will be right at the top like a genius of executive functioning, and then if you're right down at the bottom end, then you're considered impaired enough to have ADHD.
It's more like deafness - if you measured everyone's ability to hear different frequencies, most people have what we would consider a "normal" range of hearing as they are in the middle of the bell curve, some people have extremely sensitive hearing and are at the top, they might be musically gifted (or they might even be autistic, as people on the autism spectum often have hearing sensitivity). And someone with very little or no hearing ability is considered deaf, someone on the borderline between deafness and hearing might be considered hard of hearing. (I do apologise if any of these terms are out of date).
ADHD is a clinical diagnosis, like depression, in that there are no biological markers which are clear enough to differentiate - like for example you could say someone is iron-deficient if their iron is below a certain level. Although there are identifiable differences between a population of diagnosed ADHDers and neurotypical or general population, so we know it's a real thing, it's just those differences are not enough to diagnose yet (it would be like trying to diagnose if someone is male or female based on their height). So it's essentially OK, people with these symptoms normally respond to these treatments, let's try it. (And sometimes people have symptoms A and B but they can also be caused by Disorder X so let's look at that disorder too to see whether it's more likely to be ADHD, that, or possibly both). At my assessment, I hit a lot of boxes for OCD, enough to trigger a recommendation for an OCD assessment, but my assessor felt that these symptoms were actually coping mechanisms I'd employed for my ADHD rather than being caused by OCD themselves.
However the "executive functioning bell curve" model is more of a modern interpretation - previously, ADHD has been identified by looking at hyperactive or inattentive symptoms, which are basically the most externally obvious signs. Originally they thought it was two separate disorders, and only combined them into one when more was understood. But both have been identified going centuries back, so it's not a modern phenomenon. When you understand ADHD as a disorder of executive functioning then you can see the root causes of many of these more visible symptoms, but that's where we're at at the moment.