@User: Probably for the same reason that open_pdks is screwing up the install, which I just discovered, which is that all the SRAM layouts share a number of cell names. If they are not handled carefully when merging GDS files, then cells of the same name can get overwritten. In open_pdks, it's the different SRAM sizes that are overwriting each other. In the Caravel layout, I expect it's due to a revision of the SRAM layout changing the position of the cell. But that's just conjecture until I can prove it.