In the book "ADHD Does not Exist" Richard Saul describes Neurochemical Diststractibility/Impulsivity(what we call adhd) of two types : low serotoning group and high level epinephrine/norepinephrine group. "The two types described show subtly different symptom patterns.The low-serotonin group tends to look like what people think of when they think of the disorder called ADHD. The high epinephrine/norepinephrine group is "consistent in its inconsistency". Sometimes these patiens can concentrate well, and sometimes they can't." Maybe this is related to what you say, I see it myself, somedays my brain just works better, I never understood why.