My Japanese teacher, who is a native speaker, once answered a student's question about the difference between:
He said that a younger speaker might be more likely to use the second, in a context in which, for example, the third person said about himself that he couldn't speak German, but then goes on to fluently speak it, and another person remarks できてるよ！ to say "Oh see, you can speak it! You can!"
But he said that in general, he would find the second sentence to be very condescending and rude, because it implies that the speaker thinks of himself as having a VERY high level of competence himself, and judges the other looking down from that level, where as he didn't feel the first version to have that connotation at all.
Can someone explain why this might be the case? How does the ている form which usually displays habitual, perfective or progressive aspect take on such a connotation?