As your bodies fitness elevates over the course of the season you'll probably see your maximum heart rate come down. Just how much is really dependent on a variety of factors such as temperature, hydration, blood volume levels and training frequency. Over the years having lots of "athlete friends" I noticed they spoke of maximum heart rate figures as more less a badge of honor when in fact a high max heart rate is not and indicator of fitness levels. I think there was a certain psychology to that thinking and over the years knowledge about the body has dramatically changed. I had a friend whom was a great marathon runner and trained in the high heat quite often, during mid-summer he decided to have a max heart rate test in an air conditioned environment and was baffled by the lower figure than he anticipated. Training in the heat will eventually increase your blood volume and thin out your blood making it flow easier thus reducing the effort your cardio system puts out. Increases also occur in the cardiac stroke volume or how much blood the heart pumps per beat. In a well-trained heart the left ventricle fills more completely as well as produces a more forceful contraction. All these factors along with a max HR test in a cooler environment means his heart did not have to work as hard to cool the body leading to a lower max HR. The heart simply becomes more efficient at pumping. An athlete in the peak of his training can have a 3 to 7% shift in their maximum HR figures. The reverse is also true, as fitness declines your max HR increases. Due to variations with heart rates it's always more accurate to establish training zones using your lactate threshold figures if possible.