As Brovo said, ADD/ADHD is something of a go-to diagnosis before more specific things are pinned down. This is mostly because ADD/ADHD actually has a small laundry list of symptoms itself and you only need a few of them to be diagnosed.
I was diagnosed quite young with ADD for instance, I can typically keep a coherent thought going so long as I concentrate, I do however get distracted relatively easily and need to remind myself to stay on task, this is part of why my desk and monitors are littered with sticky notes.
This particular symptom is pretty much the most common but by far not the only one as many people with no experience with ADD/ADHD believe.
A second notable symptom I have for instance is the inability to filter sound, also quite common but much less well known. You know how most people say they just tune out the sound of traffic or someone talking to them, I'm actually incapable of doing that, which can make focusing on what a person is saying tricky sometimes when there a high level of ambient noise and provides another trigger to the getting distracted part.
Bare in mind of course this is just my experience with a relatively mild case of ADD, I haven't taken meds for it in about 20 years now and have learned to cope with it. there are a number of less common ADD symptoms I apparently don't show signs of, but many of those are shared with other disorders which is why ADD/ADHD is often the first thing doctors come up with to explain symptoms of a disorder.