Well, Obi-Wan never says he has never met Artoo. He says he can’t remember *owning* a droid. Alec Guinness’s smirk while delivering that line can also be interpreted as a playful lie by omission.

Perhaps he didn't own the droids? They may have just been accompanying him.

He’s being evasive.

My mental justification when I was little was that Obi-Wan admitting he knows opens the door to a lot of questions Luke didn’t need to be asking yet…

Gee I wonder