The simple explanation is: we cannot learn something we already know. Thus, when we say, "I'm learning X" it must mean I don't know X.
The problem with this simple explanation is that it's often not clear what it means to know something.
At what point do you know how to swim? When you can keep yourself from drowning? When you can propel yourself through water? How fast and for how long?
At what point do you know how to drive? When you can make the car go where you want it to? When you can pass an official driving test?
At what point do you know English? When you can speak your mind? When you know and use every word in the OED?
Since these questions can't ever really be answered, since there's no single meaning of "to know something," there's also no single meaning of "leaning something." Roughly speaking, if "knowledge of X" is a line from absolute ignorance of X to absolute mastery of X, we use "learning X" more towards the absolute ignorance end, than the absolute mastery end.