I have always had concentration problems, but I also have ADHD. It means I can't use distractibility as an indicator of anything except maybe forgetting my ADHD meds.

There is a qualitative difference between my general ADHD distractedness, hypomanic distraction, and depressive concentration failures. The ADHD mostly makes me really forgetful and liable to get up a lot instead of just buckling down to work. I also jump around a lot in conversation and writing and work tasks - basically just trying to pay attention to way too many things at once, or at least, a lot more than my brain can handle. When I'm depressed, though, I can't concentrate because my head is sludgy and slow, not because I'm trying to attend to too many things at once. When I'm hypomanic, it's a lot like the ADHD inattentiveness, but cranked up a few notches, and it's not just that I'm trying to pay attention to or do too many things at once, but I'm also having brilliant flashes of insight that I must capture, etc...

I have no idea why my brain works this way. I assume all the issues with concentration and attention are neurochemical, because I have worked so hard to compensate over the years that it's almost embarrassing. I have occasional "normal" days where it's a remarkably different situation, but those days are very, very infrequent.