there is a nutrient floor at work here. 1200 cals is the least you can eat and still get in all the vitamins and minerals and such that your body needs. so that is why your loss ranges were there.
now for exercise, you've basically added burning 100-150 cals 5 or 6 times a week, correct? which is more than before, but it's still not a ton of calories.
so your ranges are calculated by your height, weight and sex determining your average burn, adding daily activities to that and then 1/7 of the total cals you burn during exercise in a week. www.sparkpeople.com/resource/calorie_calcu
this article explains more fully if you'd like to do the math by hand.
when you're not very tall to begin with and you're not very overweight to begin with, you're not burning all sorts of excess calories by having to haul around an extra 56k of weight. when you're only a little overweight, you only burn a little more. so your losses have to be smaller and you eat closer to where you will maintain than someone with a lot of weight to lose. which means that when it comes time to maintain, you're eating close to where you were losing.