There's a difference in terminology. A false positive can occur if you're skirting the "legal limit" however, most limits are set abnormally high to remove all "false positives". Athletes that sue because they have "false positives" and think there's a physiological reason why their limit is so high are usually trying to save face.
http://sports.espn.go.com/oly/cycling/news/story?id=4915919
Floyd Landis from 2006. The typical human maintains a T/E ratio of 1:1. You can get T/E ratios up to 4:1 or 10:1 in individuals, but usually there is a medical situation involved and it's been fairly documented through your life why this is happening. A doctor screening you for competition would notice this before you began your race. But if they test you over and over and you're 2:1....then 3:1...then maybe 2:1 and all of a sudden you test at 11:1. It sets off alarm bells.
Most of these people that are doping will even do mild doping to stay below the legal limit. Keeping close to that 4:1 limit. They test it out with their personal doctors....just to see under certain conditions if they can stay close without popping.
If you're at 3:1 then you're good to go. And they can't say you're doping...even though you've jacked up the amount of testosterone you have to 3x it's normal level. Nevermind this is a ratio test....so many people will compensate by adjusting levels of T and E at the same time.