Slightly Confused

Despite what many people have said about the accuracy of this( http://www.bcm.edu/cnrc/bodycomp/bmiz2.html) calculator for teenagers, I have found that by following its recommendations, I tend to gain weight. On most days I walk for 30-60 minutues at a moderate pace. Thus, I follow the recommendation for those enagaged in low activity. Furthermore, I presently weigh around 108 lbs., am 17 years old, and have a height of 5'4". One possibility that I have considered is that, perhaps, the exercise that I engage in is not even considered light activity since I never reach my target heart rate. However, please bear in mind that I am a rather high-energy person and rarely sit still for long. Therefore, I am wondering if it would serve me well to follow the guidelines given on this website, even though they are intended for adults. One last thing: I am a girl. 

Well metabolism is affected by other things like genetics I guess, and even environmental factors such as temperature or what food we eat can affect our metabolism.

You may be underweight, and I have heard bodies that are underweight are keen to return to a more desired weight. It's something to consider.

Make sure you never go too low in calories though!