See, Joachim always seemed to have a preternatural knack for telling when Handy was not being entirely truthful with him. As a result, Handy had no choice but to be much more revealing about exactly what went on in his adventures than he had been with his knightly peers. Naturally he wanted to skimp on a few select details, the shenanigans with the Lady of the Lake being one of them. Joachim was able to pick on this and leaned on Handy to give him all of the details.