I think this is a ridiculous thing to argue about. It's like saying "Why does Obi Wan think that Darth Vader killed Anakin Skywalker? Doesn't he know Darth Vader is Anakin?" Of course he does! He is giving Luke the information that he needs to know in the wy he needs to hear it to start him on his path to being a Jedi. Hence, the round about talk of Anakin/Vader. Obi Wan obviously isn't married to the whole truth. Obi Wan never said "I don't recognize this droid." He said "I don't seem to remember owning a droid". Jedi usually only "owned" what they could carry on their person. The closest droid Obi Wan ever came to "owning" is R4 (seen in episode II, III, and The Clone Wars) but it's definitely possible R4 belonged to the Jedi order and was just Obi Wan's favorite droid to use. R2 definitely didn't belong to Obi Wan because he was always used by Anakin and then given to Senator Organa after order 66 and presumably still belongs to Senator Organa since he is being used by his adopted daughter (Princess Leia). As far as R2 saying he belonged to Obi Wan it's shown that R2 is capable of lying while conducting covert missions. This is why he is so valuable. Assumably, the easiest way for a droid to convince a stranger to bring it to the person it needs to go to is to say it's owned by that person. R2 doesn't immediately recognize Obi Wan because, really, would you? So far there is no indication that the two have interacted since order 66. The man has changed his beard style and aged 20+ years. This last point I have, honestly, doesn't prove anything but when Obi Wan first sees R2 he refers to him as "my little friend". Maybe Obi Wan is just being friendly but MAYBE considering all the things I've pointed out he does, in fact, recognize R2. I like to think so.

When it comes to c3p0, his memory was erased because, let's be honest, he's just not as coy as R2. Short of Obi Wan saying "My God! It's c3p0!" We have no way of knowing if they met before and Obi Wan is obviously playing his cards very close to his chest when we see him in episode IV and not prone to outbursts at anytime.