I have the same hrm (polar f11). Just checking... Has your weight decreased or your age increased? (by the formula you will burn less if you are lighter or have a birthday). Also, have you recently completed the built in "fitness test'? I am not sure how accurate that test is, but your calorie burn is partially dependent on that test as it measures your resting pulse and estimates your VO2Max. If you did it, did you follow the instructions: to not have any caffeine first, take it at least one day after a rest day (I think it says ideally at least two days after a vigorous cardio workout), be in bed for at least 15 minutes first, be still in a quiet place throughout the test? If your fitness has improved your score on this test may have improved in the right conditions.
But yes what others said that when you become conditioned for an activity your heart rate will not get as high so your calorie burn will be lower on a hrm (I've heard that may not be true in reality as increased VO2Max and increase muscle may raise your calorie burn but I do not know if that really balances out or not). Also, when I get more coordinated and more skilled at something I don't need to work as hard to keep up--this is especially true of dancing type activities (with dance there seems to be a sweet spot between knowing well enough to keep up and not being too efficient at it). It could be your zumba technique and coordination have improved so you are not working quite as hard.
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