I've had similar experiences - I also know some people who didn't declare small excesses, were caught and paid substantial fines.

In my case, I was once returning from Munich - had a number of bottles of wine. I declared the # of bottles and got on the long line. The agent asked me what the total was in liters - I said I had no idea and proceeded to start reading each label to add it up. He rolled his eyes, said, "never mind," and that was that.