How am I under-weight?

Hi so i did a healthcheck thing on the nhs website to see if i was a healthy weight/underweight. (You fill in your height, weight, age etc) and the reult i got was that i was a good bit under a healthy weight. But this is weird because i dont look underweight i look fairly average weight not really skinny which is kinda what ya think of when u hear underweight. So i was think seen as fat weighs less than muscle maybe i am underweight because i have more fat than mucle? like i could put on say 5 to 10 pounds but still look the same because what i gained was muscle? could some1 explain if thats what it is