So.. because alot of these muscles/ tendons are tied together..
for small independent finger movements, essentially the middle and pinky has to _Counter Move_ to stay still..
If you learn piano, there is that Hanon music book they give to beginners for independent finger exercise..
The ring finger can not move completely independent of the pinky, but minute counter_moves can keep the pinky functionally still while the ring finger moves.
YOU CAN PRACTICE..
Just lay your hand lightly curled on the desk..
Then focus on your ring finger..
DO NOT initiate a GRIP motion..
Attempt to move it in a small circle with the other fingers still.