The SYNTAX trial is a randomized trial. This means that half the patients are treated with stent therapy and half the patients are treated with coronary artery bypass graft surgery.
The therapy that each patient receives is chosen randomly, like flipping a coin. In fact the assignments are generated by a computer.

The purpose of randomized trials is to ensure the different characteristics of patients including age, associated illnesses such as high blood pressure or diabetes, and other factors are similar in the stent and surgery patients, so the two treatments can be compared. All the medication you may have taken in the past have been evaluated in randomized trials. This is the only reliable method to determine whether two treatments for the same problem are similar in effect, or if one might be superior to the other.