Parts don't wear or become crudded at the exact same rate, so it is not uncommon for one or two cylinders/pistons/rings to start acting up first. Spark plug anti-foulers are a useful way of prolonging the life of an engine, especially appropriate for an engine that doesn't rack up a lot of hours. While normally considered a last resort and sign that the end is near, many n-series tractors run happily for years or decades with them.
Wear though isn't the only reason for oil fouling. Rings can become stuck due to crud, corrosion and sitting too long. My very worn 8n used to run 2 anti-foulers. Without them it was barely useable - the plugs would oil foul in seconds especially when starting or idling, and when they did the only cure was to remove the plugs and wash them in gas. I finally tried an MMO (Marvel Mystery Oil, some prefer ATF) ring soak, and it cured the problem, so that I don't even run the anti-foulers any more.