Not to be argumentative but some situations it'd make sense:
1) they outsource the lens grinding, but either can use a patented coating of their own that the original maker couldn't, or use a publicly-available one that's better but more expensive
If the special coatings on the lens' optical surfaces changes the path of light then it makes no sense to do this.
The reality is that as the market contracts, expect to see more of this happening. Was it Sigma OEM'ing lenses for Leica? And Panasonic? Didn't Leica use an optical model from Tamon or was it Sigma?
The optical formula is what's difficult to change one part of (glass materials or coatings) because the precise design would take everything into account but the electronics, mechanisms and casing would eb easier.