Hello: The perfect anode would be a ball in the center of a spherical tank. That way it would be the same distance from all metal that needed protection. Each “piece” of the tank would need and be given the same amount of current to be protected. The internal shape of a water heater is not so simple and it’s harder to protect the ends of the tank than the center. Anode that’s closer to tank metal gets used up first. This pushes the working anode further from the tank ends over time. The manufacturers use the six inch number as the right time to replace an anode.
Rheem often puts a resistor in their anodes to slow them down. In clean water it can work too well, slowing things down too much, not giving the tank sufficient protection. It’s a tricky job, predicting anode usage as water is different everywhere you go.
Hope that enlightens more than it confuses 🙂