If you kept the same bodyfat thickness but gained 30 lbs of muscle, you would be somewhere close to 10% because it is a percentage of lean mass to fat mass. Gain more muscle without gaining fat and you effectively lower the percentage of you that is fat. Youll look bigger in the mirror but no leaner simply because the skinfold thickness has not changed.
All of this to say: use the mirror above all. The bf monitors are cool indicators, but only one tool in the toolbox. It really doesnt matter what percent you are, as long as you look 100% awesome to yourself.
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