A lower wattage element is "expected" to last longer under identical conditions, because it produces less heat. In most cases, the heat is the killer. That it is not lasting its full life suggests that somewhere, there is some problem with heat transfer. The element is not able to transfer the heat to the "point" and gets heated up beyond its rating.
Make sure that it is tightly pressed in place and scape away rust and scales from the surfaces that are enemies of heat transfer. If there is a nut and bolt to tighten, please remove that and clean and replace tightly again.