You must have just been lucky then.
I did this, just to see what happened and it was completely INaccurate; it had me out by 12% BF! I tried again, inputting measurements that I was when I was 8% BF and it was still was inaccurate, by 9% BF! I have my body composition taken by the one of the top ISAK anthropometrists in the world; therefore I KNOW that the results I get are a reliable and fairly accurate indicator of what I am. I also know that using girths is NOT an accurate measure of body composition, just the same as calculating BMI is highly inaccurate in saying whether one is 'underweight', 'normal weight', or 'overweight'.