Is this accurate?

i'm a 16 year old male and i'm 5'6 and weigh 131 lbs. and today at NFC ( NFC stands for national fitness center) i got my body fat % tested. the way i got it tested was i held onto the sides of this thing that looked like a game controller and it told you your body fat %. anyway mine came out to 21%!! i freaked is there anyway this could be wrong?? i have what i call a "two pack" i.e.: a line running up my stomach. also is it reasonable for me to drop 1-2% off my current body fat% a week? if not then what is? honest answers only please!