Seems to me that there's two separate Latin roots because we have two separate things. The net, rete, would be a saggy thing that would bring up the idea of a bag so the bag, made of the same netlike fabric, only smaller, would easily be called a "little net." The wickerwork, cratis, would be solid, possibly, but not necessarily, with a curved shape. What they have in common is the cross-hatch design of the material they are made of and this is the metaphor that came to describe the pattern on the optical instruments.