How does Paul account for Dunstan forgetting he told Paul Boy’s nickname?

From the text:

We all forget many of the things we do, especially when they do not fit into the character we have chosen for ourselves. You see yourself as the man of many confidences, Ramsay. It would not do for you to remember a time when you told a secret, Dunstan Ramsay—when did you cease to be Dunstable?”