Yep, it's what Erica said. The winner of the tests is sent later (size dependant on the % you picked). If you are just looking for A/B testing and no winning email to be sent, I'd use the champion/challenger test. Think of the challenger as your original subject line and then add one or two challengers. If you truely do have one you use most of the time I'd give that one most of the sends ~80% and the other one or two 20 or 10% each. Then decide after a certain number of runs the new champion and add new challengers. This works best for emails in nurture streams that are reoccuring.