I don't know. I've been thinking about this, and it is interesting (although probably not intentional) to think that maybe Obi-Wan didn't just lie to protect Luke, but he was also oblivious to the relationship he and Anakin had in the first place. You've seen it before, those friendships where one person is totally devoted to the other and completely unable to see that the other one can barely stand him. Maybe Obi-Wan was that delusional guy who never could tell that his overbearing advice was totally pissing off his pupil. "Oh, he's just screwing around with me. That Anakin! What a buddy!" "Um... no, I actually hate you." "Ah, there you go again!"

A generation later: "Oh, Luke. Your father and I were best friends. We were so close that he used to say things like how much he hated me and wanted to kill me, but I know he didn't really mean it. Ah, those were the days. What a rebellious little scamp. And a good friend."