It's not a silly question at all. In fact it's a topic hotly debated in weight loss circles. On the one hand some argue that this is simply the minimum of what you should consume to stop your body from going into 'starvation' mode, so it's only about the amount you eat no matter how much you exercise. Most, however, argue that it's the net calories at the end of the day which is most important, so you should have a minimum of 1200 calories AFTER you take exercise into the equation.
I agree with the second theory. You don't want to create too large a deficit due to exercise or you may be risking what they call exercise bulimia. Your body needs a certain number of calories to keep itself running. Keeping your organs functioning requires about 700 calories per day, anything less than that and you will starve to death. That is bad. Between 700 and 1200 calories is the 'starvation mode' zone, where your body can keep itself going but thinks it's in a famine. This slows your metabolism and makes you burn fat at a lesser rate than before. That is also bad. Between 1200 and a figure determined by your current height and weight is the safe number of calories for weight loss. That is good.
Now consider if you are eating 1300 calories per day. Your body is happy with that. It has enough calories for basic functions and enough left over that it's not flipping out and preparing for an imaginary famine, but there is a sufficient deficit between this number and your weight maintenance calories that you are losing weight as a steady pace. Now throw in 400 calories worth of exercise. Instead of having those calories for your body to play with as it sees fit you are 'throwing them away' on physical exertion (your body's perspective, not mine!).The sum of your calories at the end of the day for your body's processes is now in the starvation mode range. So naturally your body flips out and puts you into starvation mode. Whether your caveman body is lacking in food because there simply isn't enough food to eat or because you are having to spend an extra 400 calories hunting or foraging to get your normal amount of food means little to it. All it knows is that you are having severe problems with keeping your energy intake adequate compared to your body's needs so it adjusts to make sure you don't lose too much weight and starve to death.
Others may have other opinions on this, and it would be best to consult a fitness or nutrition expert for their take on this. But this is how it makes sense to me!