Down 3.6% body fat!

You are already at a healthy weight. So running will most likely be burning off more lean muscle mass than fat mass. Unless you are doing weight lifting and strength training.

People with a lot of weight to lose will more fat than muscle.
People at healthy weights will lose more muscle than fat (the muscles will get smaller.) unless you are strength training with the cardio. (Dieting also causes muscle mass loss)

Check out: "An Email Response that Might Help Some" in my signature that sort of explains it.

Where are you getting the fat loss number? I have a digital scale that measures body fat and the accuracy can be off as much as 3%.

And I love this quick article: http://www.hussmanfitness.org/html/TPAdaptation.html

The law of unintended consequences

Your body is an amazing feedback system aimed at balance and survival. Humans are at the top of the food chain because they are able to adapt to their environment. Every action produces a reaction. Every change in its environment triggers a survival response. It's important to keep that in mind when you plan your fitness program. If you treat your body as an enemy to be conquered, you'll produce unintended results.

For example, if you severely cut off the supply of food to your body, it will defend itself by slowing down its metabolism to survive starvation. The body will shed muscle mass the same way that you would throw cargo from a plane that was low on fuel, and it will reduce its thyroid activity to conserve energy. The body will also actually defend its fat stores. In anorexia, muscle loss can be so profound that fat as a percentage of body weight actually rises. Extreme carbohydrate restriction also causes muscle loss, dehydration, and slower metabolism, which is why even successful Atkins dieters can have a significant rebound in weight after they stop the diet (don't worry – the advice on this site will prevent that from happening).

As another example, if you put your body under stress through overexertion and lack of sleep, it will respond by slowing down, reducing muscle growth, and increasing your appetite for junk food, carbohydrates and fat. If you feed your body excessive amounts of sugar and quickly digested carbohydrates, and it will shut off its ability to burn fat until those sugars are taken out of the bloodstream.

This website will show you how to work with your body to quickly produce the changes you want. In order to do that, you need to take actions that push your body to adapt – to build strength, burn fat, and increase fitness. You need a training program, not an exercise routine. You need a nutrition plan, not a diet. You need a challenge, not a few good habits you usually try to follow except when you don't. (Don't know how great the website actually is)

Our bodies defend our fat reserves and will try to burn off something else (especially if there isn't a lot of body fat) usually muscle.