Individual metabolism varies greatly depending on age, how long sedentary, how long overweight, and so on. The formulas just give a number based on general assumptions.
To get FitDay to work for me, I have to set it at completely sedentary. Then I add any exercise I do to the Activity Log. That comes out pretty well--but at the moment, the calories I burn during 24 hrs. comes out to 1,786. I add about 200-300 with exercise. That allows me to eat more than 1200 cals a day.
Just for comparison, my BMR from that website comes out to 1,384. It's a good thing I don't stay in bed all day!
So, just remember, no number is engraved in stone, and weight loss is not a straight line.
Jay