There's two answers to this. I could write pages on this subject, but will try to keep it short because I'll probably just end up making myself sound like a prick...
1 - Eating more carbs is not going to directly force your body to burn more calories. Depending on the types of carbs you're talking about, you could be seeing the effect of insulin, among other things....
2 - Low carb diets are flat-out BS and I'll call anyone to the carpet who says otherwise. In a nutshell:
- they do NOT force to burn more fat than you would otherwise
- they ARE very detrimental to anyone engaged in above average physical activity (ie: not sitting on your a$$ watching TV all day)
- they DO force your body to metabolize and break down muscle protein at an astonishing rate
- they ARE counterproductive to AAS cycles
Your cells REQUIRE glucose/[glycogen for storage] for energy production. If you are not taking in enough carbs (which are broken down into glucose), then both fat and muscle needs to be broken down in its place. What does taking in enough carbs mean? About 60% of your calories should be from carbs. The tougher question is how many calories you should be taking in.
So what happens if you're buying-in to the crap information that corporations like Atkins would love for you to believe? Since you're not taking in nearly the amount of carbs you need, you body will break down muscles to get to glucogenic amino acids. Yeah buddy - up to 90% of the glucose required by the body to support energy production in working muscle cells, brain cells, and nerve cells can come from the destruction of muscle. The actual distribution of % glucose from protein vs fat depends on diet and other factors such as how beat-up your liver already is (not to mention the downward spiral you're putting it on by combining a high-protein, low-carb diet with AAS). What's even funnier is that all the other amino acids (and nitrogen!) from the newly broken down muscle protein just gets disposed of, so it's a complete depletion and waste of muscle!
Does fat get burned in this process? Sure, some does, but you'd be better off keeping your existing muscle (or putting more on) which in-turn jacks up your resting metabolic rate, as opposed to sabotaging it. All of this is why many people see % body fat not change, or in many cases go up, when they do these diets. Of those who do see a drop in % body fat, it is always because of cardio and other factors, and I can guarantee you they have dropped a considerable amount of muscle along side that fat, as opposed to probably putting on more like they would have done with a healthy diet.