Okay, it's possibly been done to the deathalready but here goes.
I've seen the 'why doesn't 3PO recognise Ben stuff, but has anyone asked why Owen and Beru don't recognise R2D2. Or maybe R2 units are easy to mix up? Oh My God - the Lars are droidist! A bad case of 'They all looked the same to me'?
Okay, any other answers; daft or otherwise