Yes. Heart rate can climb on race day while wattage is always the same day to day. You can think of it in terms of weights in a gym. A 45 pound weight is 45 pounds every day, every place, all the time. This helps on race day because you know from your workouts and training what wattage workloads you can sustain for a given amount of time. The is a key is being able to pace yourself more effectively.