How come I'm not losing weight?

To be clear, it’s not that I need to lose weight, it just seems as if I logically should be. And I wouldn’t mind losing five pounds : )

Here’s the deal: I’m a small female. I was fooling around on some health and fitness website today and they said that to maintain my weight, I need to eat 1900 calories a day. But the thing is, I usually only eat 900–1200 calories a day. That’s just how much I naturally consume. And they say that 3500 calories equal a pound.. so at that rate I should be losing like at least a pound a week, but I’m obviously not. I’ve been at a very stable weight for two or three years.

I get some exercise, not a lot but I walk or bike 2–3 miles every day. And like I said I don’t have a large amount of caloric intake, and it’s all very healthy food. So what’s the deal?