I see this as consistent. When you say "I play the piano" you are really saying, "There are many instruments – banjo, trumpet, clarinet, and piano. I play the piano". Saying "I play a piano" is also correct. But the inference is that you are referring to one particular piano.

In the sentence "There is a piano in the lobby" you are saying "There are many objects in the lobby. Among them is a piano, one of many in the world".