As far as I understand, the Carotid Intima-Media thickness grows by the age and by diabetes. This is not necessary due to atherosclerosis that I think only affects to the intima.
Although it is not fully understood, why this happen, keep in mind that the structure of our arteries are different in different parts of the body and one of the factors that influence the NO production by the intima, which is a major cleaner in our arterial system, is the share effort of the blood against the artery wall, which in turn depends of the diameter of the artery and velocity of the blood.
So it is not understood, but it is clear that the risk for atherosclerosis it is not the same everywhere.
In fact if what you look is at the risk of heart attack due to atherosclerosis in coronaries, you should look at the CT scan as a more reliable indicator