The way it was explained to me, and it totally made sense is that 1200 is right for some people but not for everyone. Many peoples basal metabolic rate is higher than this, this is the amount of calories you would need to stay alive if you were completely sedentary, say in a coma or something. There is a calculation that you can do to work this out by the way if anyone is interested. So, if you are eating less that that bmr, your body thinks it's in starvation mode, and whilst you will initially drop some weight, eventually the body is going to go into panic mode and stop losing.
So at the very least you need to be eating at this bmr, in fact I have found that by using the calculation to work out the maximum you can eat without losing or gaining weight, then dropping the calories by 15-20% I am still able to lose weight which makes me very happy.
Blimey I really hope this makes sense lol