Calories in, calories out is indeed how it works, but people also need to realize that your body is not a static machine, it reacts to its environment. Restrict caloric intake too much and you hit a point where your body enters a 'starvation mode,' it holds back on as much work as it can and holds everything it doesn't use. If you have problems absorbing foods due to problems with your GI tract, you might find that you don't need to restrict too much before that happens because you weren't getting all the calories you thought you were before.

Of course, reducing calories will cause your body to change some of what it does even if you have no other issues, so for maximum benefit, you should pair it with exercise.

I've also restricted myself to 1500 calories a day for the past month or so and have lost about 10 lbs so far and am on track to lose a similar amount in a similar time as yourself. My wife did the same thing at the same time, it's not going anywhere near as smooth for her, but she has GI issues.