Gaining weight on a diet?

Two slices of wheat toast for breakfast, fruit for lunch, more toast in the afternoon when i get home from school, sometimes with some yogurt or a banana, then dinners kept around 600-700 calories and I try to get all the food groups in there, Monday thru Saturday. Sundays are cheat days, but it's not like I go out and eat a mountain of lard. Then I get a bit of light exercising in Monday thru Friday. I've been sticking to this diet for 10 days and I just weighed in and gained a pound? Any explanations?