The droids should have recognized Lars and his farm as well, since they've both been there. And Lars should have recognized the droids in ANH, at least C3P0, who he spent a great deal of time with.
Maybe C3PO gets a memory wipe. I can see R2D2 not getting a memory wipe, that droid seems to know what's going on in ANH, and where to find Ben, etc. But 3PO doesn't know anything.
This seems likely since it appears to be common practice to wipe droid's memory when they switch owners. Plus 3PO's memory may have been wiped for security purposes by those who wanted to hide Luke and Lea from Anakin, or by the rebellion later on.
The part that bugs me is that Lars doesn't recognize the droids. Sure there are plenty of droids around that look like 3PO and R2, but once he heard the names, particularly C3PO, it should have jogged his memory...after all the guy lived with 3PO for nearly a decade prior to AOTC! How could he forget a droid that he lived and worked with for nearly a decade?