In the prestige movie, why does Borden keep saying (in the diary as well as to Angier) that he doesn't know which knot he tied at the Julia drowning scene?

Its got to be one of the twin brothers who tied the knot and one of them would know surely know which knot they tied?

In "The Prestige", why does Borden keep saying (in the diary as well as to Angier) that he doesn't know which knot he tied at the scene where Julia drowns?

It's got to be one of the twin brothers who tied the knot. One of them would surely know which knot they tied.