It's not really a matter of memory. I think it's described that way so that we can understand it better. Dogs are the ultimate "zen critters"--they live more
in the now than any of us could hope to. We can't explain to them, "Okay, I'm really upset that you pooped on the floor three hours ago. That's why you're in trouble." Because we speak two different languages and we can't accurately communicate our motivations, we have to take advantage of their doggy zen and use it to teach them. That's why, if you want to shape your dog's behavior, you have to react to their behavior within about four seconds. Otherwise, it's in the past, man!