There are multiple ways in which either the estimate of calories required to maintain your weight or the estimate of how many calories you've consumed or expended can be persistently biased by an unknown amount. If you maintain a positive (negative) net calorie bias, you'll gain (lose) weight. If you can't know the net bias (due to the unknown biases listed below) you can't predict whether you'll gain or lose weight.
BASAL METABOLIC RATEhttp://www.nal.usda.gov/fnic/DRI/DRI_Energy/energy_full_report.pdf
Dietary Refernce Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, Panel on Macronutrients, Panel on the Definition of Dietary Fiber, Subcommittee on Upper Reference Levels of Nutrients, Subcommittee on Interpretation and Uses of Dietary Reference Intakes, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine of the National Academies
"By definition, the estimate would be expected to underestimate the true energy expenditure 50 percent of the time and to overestimate it 50 percent of the time, leading to corresponding changes in body weight. "
"Sedentary means a lifestyle that includes only the light physical activity associated with typical day-to-day life. Moderately active means a lifestyle that includes physical activity equivalent to walking about 1.5 to 3 miles per day at 3 to 4 miles per hour, in addition to the light physical activity associated with typical day-to-day life. Active means a lifestyle that includes physical activity equivalent to walking more than 3 miles per day at 3 to 4 miles per hour, in addition to the light physical activity associated with typical day-to-day life."
Obesity Prevention in the Information Age: Caloric Information at the Point of Purchase
Mark Berman; Risa Lavizzo-Mourey
"most individuals significantly under- estimate the caloric content of restaurant food, espe- cially for higher-caloric foods."http://onlinelibrary.wiley.com/doi/10.1111/j.1753-4887.1990.tb02882.x/abstr...
How Accurate Is Self-Reported Dietary Energy Intake? Dale A. Schoeller
"comparisons have been made in nine recent studies, and considerable inaccuracy in self-reports of energy intake has been documented. Reported intakes tend to be lower than expenditure and thus are often underestimates of true habitual energy intake"http://www.ncbi.nlm.nih.gov/pubmed/11641742
Validity of self-reported energy intake in lean and obese young women, using two nutrient databases, compared with total energy expenditure assessed by doubly labeled water. Weber JL, Reid PM, Greaves KA, DeLany JP, Stanford VA, Going SB, Howell WH, Houtkooper LB.
"Both physically active lean and sedentary obese women under-reported TEI regardless of database ..."