I agree with Hmoll completely. To add to this point. Many pianists seem to have this thinking that once they learn a piece and can play it in time, they have 'mastered the piece'. I guess this is a normal human nature, many young pianists seem want to rush and play as many pieces as they can, never bothering to sit down and work out the music from the pieces they are learning. Just go to any music competitions nowadays, most of the pianists play 200% correct notes, and 0% music. As a general rule, once you get the notes and tone right for the piece, you will need to take at least double or triple the time taken for learning the notes to work out the music. Most pianists don't see the point of spending a lot of times working out the music. Compete with yourself to make better music, and communicate often with your teacher to gauge your music making capability. You may think you master a piece well, but a teacher can always find some constructive feedback for you.