High body fat percentage but thin!!!?

i'm 155cm and weigh 49.3kg...which i think is about 5 foot one and 7 stone 10....but i did a body fat percentage test the other day and it came out at the very very top of the "normal" range...i appear thin, a UKsize 6/8 (american 4/6)...but why would body fat percentage be in the higher bit of normal...surely that would be what "chunky/bonnie/biog boned" normal people get....whereas i'm really quite petite with not much apparent body fat (small , etc)....is it something i should try help - like curb my diet? i eat a lot of fish, oats, spinach, chicken, flax seed.....don't drink milk/eat cheese/cream products nor sugary foods...drink abotu 1-2 espressos per day + 1 litre of water