It's believed the sandals are providing more arch support as they wear down. 

A bump tends to form between the heel & toes.  It was originally believed the bump didn't provide any arch support, but it could be supporting the ends of the arch & that's affecting the longevity of the post tib.  The front & rear are more compressed, so are supporting that part of the foot more.  It's possible the layer of fake leather is reducing the support from the bump.

It might be necessary to keep sandals going longer & use only older ones for the highest effort.  Some say arch support is bad because it weakens the post tib.  No matter what, all cushioning of any kind is going to unload the post tib more as it ages.  The only paw wear that doesn't is bare rubber. 

The sandals showed wear shifting to the rear, after 4 months of switching to a heal strike.