I think it's more a matter of, if he'd been caught, he'd have said something along the lines of, "Oh, oops, I don't know what I was thinking"--and mentally marked down "this is not an ideal person to con." When the person he was conning either didn't bother to verify what he was claiming, or insisted that a minor difference of fifteen years must have been a sincere misunderstanding instead of a deliberate lie, then he knew he'd hooked a new victim.