explanations for increased NO measurements

My 2000 4-cyl Camry LE just passed its bi-annual California smog check, but the NO reading went from 0 two years ago to 229 at 15 mph and 115 at 25. Still way below the legal maximum, I'm wondering what may have changed over the last two years. Other than a new timing belt and oil, maintenance has been virtually zero. The car averaged almost

Thanks, Herb