After a (very) dirty bulk, I have now been losing weight for about 39 weeks. (It has been going OK.) At the start of my diet (at the end of November, 2015) I was about 110 kg. I planned my weight loss proces to be slow and long, so as to maintain as much muscle as possible. As a guideline, I used several 'total daily energy expenditure' (TDEE) calculators to find a baseline for my diet. All of the calculators I used gave me a TDEE of about 3,000 kcal. (Of course, including exercise, cardio, etc.) I eventually cut down to a weekly average caloric intake of 2,500 kcal per day. This, given the calculated TDEE level of 3,000 kcal, should be an average deficit of 500 kcal per day, which was the deficit I was looking for, and planned to maintain for as long as necessary (until I was happy with my bodyfat level).
At the time of writing (late August, 2016) I am about 90-91 kg. Suppose for the sake of simplicity that I have now lost 20 kg, since the start of my diet. That is an average of about 0.5 kg per week, which is fine. Assuming the '3500 kcal per pound of body fat' rule is true, and that my weight loss has been mostly due to fat loss, we can calculate that my daily caloric deficit should be around 500 kcal (it is actually more like 560 kcal).
Using 'myfitnesspal.com' I have logged my daily caloric intake and expenditure due to cardio carefully. Over the course of my entire diet I have calculated that my average daily caloric intake is about 3,000 kcal. Taking the above calculation into account, that would give me a TDEE of about 3,500 kcal.
Where does this discrepancy come from? On the one hand we have a TDEE of 3,000 kcal, based on age, weight, height, 'activity level', etc. On the other hand, we have a measured TDEE level of 3,500 kcal. How can this difference be explained? Is it common that the online calculators are this inaccurate? Or is it simply because my measurements (e.g. actual caloric content of food may not be as said on the package, caloric expenditure due to cardio may not be as calculated by the treadmil) are too inaccurate?
As some added information which may be important: even though my measured average daily caloric intake is about 3,000 kcal, this is not actually the amount of calories I ate daily. During weekdays I would always eat around 2,500 kcal per day. On training days this would make a net intake of 2,000-2,200 kcal, depending on the amount of cardio I did that day. During the weekend I would then 'cheat' (not really, because it was incorporated as part of my diet) by eating a lot more. The amount of calories I ate during the weekends would vary a lot; sometimes it was around 3,000 kcal, sometimes 4,000-5,000 kcal or more. Could the discrepancy be explained by the erratic daily caloric intake? I can imagine that the weight loss by such an erratic eating pattern would be different than for a diet where the caloric intake is exactly the same each day.
Thanks in advance!