Do really know how to play the piano?

Most people that play the piano probably think that they can play anything anytime. Not so, try playing accompaniment piano, and the rules change. They change because you are not playing alone.
When you play accompaniment piano with a person, that is singing you will need to learn to change your style. This is what most people don’t realize when beginning to learn to play with someone else, and not alone.
Your techniques will change, and you will have to find what fits best for the person that you are playing accompaniment piano for. Every person has a different voice, and the way they use it while singing is what you need to adjust you techniques to.
It is a different type of playing, one that connects you with that person you are playing the piano for. So, most people that play the piano, really don’t know how to play accompaniment piano. It is something that you need to practice, routinely.