Calories too low for outdoor bike ride

I have recorded an outdoor walk of 50 minute or so on my Apple Watch SE as being around 132 active calories for 50 minutes with an average heart rate of 91 bpm for heart rate. Today I biked for 37 minutes (only 13 minutes less) and my calorie count was 124 active calories with an average heart rate of 136 bpm for heart rate. It just seems low for such a high heart rate. I was sweating heavily during biking, not during walking much yesterday. I’ve noticed this more than once when biking outdoors. Anyone figure out why?