When you connect B to A, you also connect A to B! Which means that if you enter A, your letter turns to B, and the letter that comes back can be anything but B; but the only way to get an A as a result is if the letter going through the plugboard is a B! Which means that by extension, you cannot get A in return.
When you connect B to A, you also connect A to B! Which means that if you enter A, your letter turns to B, and the letter that comes back can be anything but B; but the only way to get an A as a result is if the letter going through the plugboard is a B! Which means that by extension, you cannot get A in return.