21 is the amount of assists across competitions, for the league alone his tally stands at 13. 5 of the remaining 8 came from the two matches vs Legia, though that's not to say he doesn't deserve those stats.
I also don't get the surprise. Both he and Mkhi are/were two high quality play makers being utilized to their fullest potential by giving them sort of a free role in a very attacking team . Mkhitaryan being in the form of his life probably also helped a bit with the numbers.
For United Mkhitaryan plays a different role in a different tactical setup, so naturally he looks worse.
Dembele would probably cope better because with his dribbling skills he's better suited to a setup where players get less support and are expected to make things happen on their own.