Perhaps they were just assuming that a differential between interest in new/old objects will always exist, regardless of factors that shorten/lengthen examination time of all objects (presumed to do so roughly equally).

Yes; that's what I expect. But making that assumption, and then focusing on just a small piece of the data they collected, caused them to overlook the large differences in time spent examining new objects.

And they could be right in that assumption! It just seems very unlikely. I think it's more likely that someone startled the rats when they were being exposed to the new objects at the 60-minute mark. For instance, one of the lab technicians visited his girlfriend that day, and she has a cat.