First assuming this is an aortic root measurement. Difference in values are the fact that measuring the size isn’t a perfect science and as any measurements you will have some error. Though 3 mm is a bit of a difference not sure what wrong there. The new recommendation for surgery is changing of .5 cm/y or above 5 cm. This is because the risk of aortic dissection increase quite significantly after around 5.5 cm. The risk of surgery is around 5 percent. They will perform a risk analysis based on the information the patient gives. The surgery will only be performed if the risk of surgery becomes lower than the risk of dissection. Ie) risk of dissection with current measurement > risk of surgery. If surgery is successful then a long happy life dissection free is quite normal.