Electrodes tend to wear on one spot from two primary things.

One is a substantial difference in electrical resistance. Not common with plugs like NGK, but quite common with things like Splitfires where one spot is noticably closer than others.

The second reason is swirl patterns in the combustion chamber. Some engines are very consistent in their swirls, as well as strong, and have the spark plug placed right in them. This is actually a good thing as the combustion process tends to be quite good in those engines. It results in a very specific and consistent wear pattern on the spark plugs.