It gets more complicated, not all anodes are created equally, different material for different water types.
Kapiti's water amongst others (when our bores are running during dry weather) is classed as aggressive. Therefore a stronger anode is required, it's been a few years since I've looked into it, and I may be wrong, but Rheem make 3 types of anode, color coded, I forget exactly, but magnesium for normal water and a solid aluminum anode? (this is where I may be wrong) for aggressive water, and some random alloy mix for in between water.