As a matter of law, "ownership" and "control" are two distinct concepts.  Similarly, "power" and "right" are two distinct concepts. 

If I have car keys, I have control over the car associated with that key.  That is to say, I have the power to drive it around.  Whether I have ownership over the car depends upon a panoply of factual circumstances that are as varied as every day life.

Similarly, if I possess a private key, I have control over the coins associated with that key.  That is to say, I have the power to dispose of them.  Whether I have ownership over them depends upon a panoply of factual circumstances that are as varied as every day life.