@Akiras @BananableOffense
Yes good question: this can be due to a few things like light polished or used/worn out material. In this case you can have a sharp lobe and a rougher lob due to damaged part of the surface for instance (concretely this is a lot more complicated but it is a good approximtion).
And example of paper discussing it is https://inria.hal.science/hal-01818666/document.
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