Will my body get "used" to my daily activity level?

I burn anywhere from 250-600 calories a day not including my excercise according to my Fitbit. Now, I've recently been reading that your body stops burning calories after doing the same activities, should I stop eating back the calories burnt from normal daily activity? 1200 calories just seems soooo little to eat. I also lift 3 times a week and do cardio 5 times a week, and have been trying to switch up my cardio every day to "confuse" my body. The idea that my body will stop burning as many calories just seems to go right over my head. There are so many people here with great advice, figured I would ask.