Why would one Carotid Artery be blocked but not the other. I know someone who has a 50% blockage on the left side, but minimal on the other ... why would this be?
The amount of blockage depends on factors that are somewhat random as they depend on many things that can sometimes be more in one artery and less in the other.
Plaque that accumulates on the inner walls of your arteries is made from various substances that circulate in your blood. These include calcium, fat, cholesterol, cellular waste, and fibrin, a material involved in blood clotting.
The amount of substances could vary per blood vessel, but more plausibly how they respond.
In response to plaque buildup, cells in your artery walls multiply and secrete additional substances that can worsen the state of clogged arteries.
Hypertension is a risk factor for Carotid Artery Disease and how hypertension damaged vessels can differ. Hypertension and Artery Damage explains how through Hypertension microscopic tears turn into scar tissue that facilitate the building of the plaque blockage. The effects of the high blood pressurw would not be the same for every artery depending on size, and other variations.
The randomness of these events is the first thing to be considered. In facts the question could be changed in "why all arteries with the same calibre aren't blocked in the same way?"
Moreover there are anatomical considerations because there are different pressures and fluxes in different points of the circulatory system, so if there is a different fluid's motion in a point it's more probable that it'll start forming the plaque.
Last thing to consider is that once the plaque "building" is started in a place (also at not detectable levels) it'll be more likely it will grow more in place that another plaque start in another place.