Because we're a bunch of liars:
It's been reported in the New England Journal of Medicine that people attempting to lose weight tend to underestimate the amount they eat by as much as 47 percent and to overestimate their physical activity by as much as 51 percent. When scientists at the USDA's Beltsville Human Nutrition Research Center in Maryland asked 98 men and women how much they ate in a 24-hour period, they found that 6 out of 7 women underreported by an average of 621 calories, and 6 out of 10 men underreported by an average of 581 calories.
Keep that in mind next time you hear someone explaining that, though they haven't let a single morsel of food pass their parched lips in days, they've nevertheless gained three pounds that week.
