This is what's known in my profession as a "bug." Not only doesn't it seem like it should work that way, it really shouldn't work that way. The intention is that the higher ablation level (the durability as you call it) should take precedence with duration breaking ties. Please submit a bug report on this to make sure it doesn't get overlooked. Feel free to quote me in it. This doesn't strike me as a quick fix, but I've been wrong before.
~Silthe