They could have used whatever was around that was as fast or faster then needed (but not slower).

I have seen video cards that had different chips and speeds before, but it is not common (they ran out of one batch and started using another in production).

If even one of those chips is 100NS then that is the fastest that module can go. I have never seen varying speeds in Apple VRAMS as of yet, and I snag any that come my way.