You have to burn 3500 calories to burn 1 pound of fat.
Here's an example: Each day, you burn a certain number of calories - let's say 2000, with exercise - and on your diet you ingest about 1500 calories. That's a 500 calorie a day defecit (1500 Kcal IN - 2000 Kcal OUT = -500 Kcal).
So, after 7 days of losing a net 500 calories a day, you've lost a total 3500 calories, or 1 lbs of fat.
Now if on day 7, you eat pizza and have 3000 calories in that day, you get a surplus of 1000 calories (3000 Kcal IN - 2000 Kcal OUT = 1000 Kcal surplus). This takes the 3000 calorie deficit you've built up during the week down to 2000 -- delaying the time it takes to burn off that pound by another 3 days.
So, this way it takes 10 days rather than 7 days (remember, just an example) to burn a pound of fat. Small difference? Over time it amounts to quite a bit.
If you stick the diet, you can lose 4 lbs in a month.
If you cheat too badly, you'd only lose 3 lbs in a month.
By the same token, if you stick to the diet, you'd lose 48 lbs in a year.
If you cheated too much, you'd only have lost 36 lbs. Big 12 lbs difference.