Does anybody know why scientifically your body burns less calories the more you exercise? (Without weight as a factor)

I've been off from working out in late Nov and I started up again the End of December. At the start I could easily get my heart rate up to 180bpm doing cycling for cardio keeping my wattage around 150 for 10-15mins. I'd also lift weights for about an hour and my exercise calories according to Fitbit (I know it isn't 100% accurate but if I compare the same numbers from the same device then I get an overall trend regardless of the numbers) burnt was about 300-400cals after a session . Today its been about 4ish weeks of working out 6x a week. I'm barely getting my heart rate into the cardio zone so its around 150-160bpm despite me having the same 5min cardio warm up and increasing the time of cardio from 10mins to 20min; keeping the wattage output on the bike the same at 150 watts. The weights I lift are getting heavier with the same sets/reps and the total weight lifted during that time is also increasing. At the end of it my Fitbit says I'm buring 200cals max despite the increase effort.

I get this is my body adapting to the exercise but is there an explanation to how the body is adapting to the exercise because if you break it down to the basic physics I'm still putting the same, if not more due to the increased time, amount of "work" as in joules of energy into the bike; which can be calculated with the time and average watts on the bike (I'm using the same bike at the gym so even if the power meter is off by some degree of error I'm still maintaining whatever that true wattage is from the start.) Weight loss isn't really a factor into my calories burn, I'm assuming, because I'm not doing cardio that is directly effected by the amount of weight on me, such as running. The amount of energy should be increasing with the weightlifting although I'm not sure how to calculate the kilojoules of energy used. It's not going to be nearly as much as cardio but I know it has to be increasing with the progressive overload I'm doing with the weights.

submitted by /u/pwn3dbyth3n00b
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