Because you increase the release of THC metabolites from fat tissue with aerobic exercise in principle. This is good if you drug test is 5 days away or more. These metabolites are released throught the kidneys and liver. When you get down to the last 2-3 days before the test, you don't want to release extra metabolites by burning fat becasue they will end up in the specimen cup. They have to go somewhere.

Does that make sense?