It sounds like they used the Futrex 5000 device on you the second time ... It's an infared device and it's accuracy depends upon the skill of the person operating it.

The best way to get accurate reading for bodyfat % is to use the same method before and after, and to try and use the same tester as well (trainer, etc.)

Having said that, all metods have a 2% margin of error (+/-), so if you were 21% and dropped to 13.3% it COULD be (in reality) a drop from 19% to 15%, which is still a sizable drop. 4% in a month is pushing it ... 7%? I am sure it is possible, but I haven;t seen it. That's a huge drop.

13% is pretty lean and mean for a woman, so you can tell if it is accurate by judging from the visibility of your abs. At 13%, you should be seeing the outlines of your obliques, as well as the top 2 or 4 abs clearly when you flex them.

If this post comes across wrong, I don't mean it to. I am very very tired, and I am just presenting the facts as I know them.