Because George Lucas doesn't have a clue, it really can not be made sense of in any other way.
It just doesn't make sense, because:
1) The only reason Owen would not recognize C-3PO is if 3PO is, as suggested, one of a mass-produced series of droids. However...
2) C-3PO was not part of a mass-produced line, he was hand-built by Darth Vader.
3) He really should've had a clue when R2 started talking about Obi-Wan, the guy who OWNED the droids last time he saw them, but he just gives some vague response when questioned by Luke, rather than desperately trying to get rid of the droids.
4) C-3PO should have recognized Owen and Beru, unless his memory had been wiped. However...
5) He remembers his first job, so his memory has not been wiped.
6) So, how do the droids not even remember who Obi-Wan is when the message is recorded?
There really is no decent explanation beyond rushed writing and production of the movies so that the associated toys could hit the shelves.