Turbulence brings oxygen into the water and it is the oxygen that causes corrosion. The forward edge of a keel, or bow for that matter, seperates the water but causes little or no turbulence. As the water flows along the surface, turbulence is induced by small imperfections in the surface, and the further aft it flows these eddys interact with one another creating a greater effect as you go. At the trailing edge, seperate flow paths meet and this is where the greatest amount of turbulence is generated. The oxygen molecule is unstable and extremely desirous of bonding with something. The sacrificial anode made of zinc in this case bonds more readily than the steel of the keel or centerboard.
Zincs are always placed where turbulence is greatest. Hence they are found on rudder posts, the rudder blade, and in the area of the screw. On a steel hulled vessel they are found virtually nowhere else other than at or near the stern.
It sounds like your centerboard is pretty well protected in terms of coating. You should bear in mind though that what appears well coated to the human eye may not be well coated to the indefatigable corrosion process. Your first clue may be bubbling of the coating or it may just work away beneath with little outward signs. The easiest way to determine if these factors are present is to mount a small zinc and see how long it lasts. Further action, or inaction, can be based on your results.