I would say the plugs were changed so the hotter heat range plugs are used to stop fouling because mowers run at a lower RPM 2500 to 3500 compared to car petrol engines.
Conversely, hot plugs are good for applications that operate mainly at low rpms. Because they have a longer insulator nose length, heat is transferred from the firing tip to the cooling system at slower pace. This keeps the spark plug temperature high, which allows the plug to self clean and prevent fouling.
