Strong coffee can have a paradoxical calming effect on people with ADHD. I have seen this effect. The kid is all over the place but four cups of coffee later he is able to sit still.
Not so strange - almost every drug for ADHD is a serious stimulant!! (as in controlled substance level)
There is some evidence that ADHD is really caused by one part (gad, I can't remember what part) is already running as if it's on stimulants, and the part for impulse control is NOT. The therory is that by dumping stimulants into the person, you speed up the part of the brain that does impulse control to match the other part
Disclaimer - My son has ADHD, and I really should have my info more at hand, but I don't. He is gradually getting himself under control more and more without his meds, which is fairly common