The digits sit one row further away than the letters, so the hand has to rise a little to reach them. The columns stay the same: every digit belongs to the finger whose column it falls into.
a s d f → 1 2 3 4
Digits are the one place where most typists really do look down, and also the place where looking costs the most: the eyes leave the screen and when they come back the place is gone.
When there are many numbers at once, a separate numeric keypad is faster. For single digits inside text the top row wins, because the hand does not have to go anywhere else.
The drill uses digits in no particular order, so the hand cannot rely on the next digit being a neighbour of the last one.