wrote:
Does anyone know how much the difference is? And better
yet, why is there a difference?

From a NYTimes article posted to ST:

"The test starts with an isolation of testosterone from the athlete's
urine. Then chemists determine the makeup of the carbon atoms that form
the backbone of testosterone.

Ordinarily, carbon atoms are made up of six protons and six neutrons,
giving them an atomic weight of 12. But occasionally, they have an
extra neutron, giving them an atomic weight of 13.

By chance, soy plants are the source of most pharmaceutical
testosterone. They tend to have slightly less carbon-13 than other
plants that are more abundant in the human diet. Humans make
testosterone from the food they eat, so their testosterone typically
has more carbon-13 than the testosterone that drug companies synthesize
from soy.

But these differences are tiny.

The test determines whether the testosterone in the athlete's urine
has less carbon-13 than another naturally occurring hormone in the
urine, like cholesterol. The test is considered positive when the
carbon isotope ratio - the amount of carbon-13 compared to carbon-12
- is three or more units higher in the athlete's testosterone than
it is in the comparison hormone. It is evidence that the testosterone
in the urine was not made by the athlete's body. Landis's
difference was 3.99, according to his own doctor."