I ahve a friend who is a brilliant piano player but just doesn't 'get' how a guitarist etc can just 'busk along' in a session and work out chords as they go along. Yet she, given the dots, can work out chord sequences lightning fast, for a keyboard and thusly any chord playing instrument. I suppose it's what you're used to.
I had to have the chords written down when I started playing guitar but eventually my ear got trained enough for me to be able to work out sequences nearly every time. There are times though...
And I can't do what Richard B does and make a chord up just like that.