Well, in fact they didn't. Because that (60Gbps through 40Gbps bus) would be literally impossible.
As I said, this thing works like a USB hub: you can get 4 ports out of one, but they will share the bandwidth of that one port. If you connect 4 hard drives to a USB hub and try to read from all of them at once, you eill only get 1/4 of the real USB speed for every hard drive.
Similarly, if you connect everything to a thunderbolt port, the individual pieces will get only 2/3 of the intended bus speed. They made all those choices available, but you have to choose which ones you use and which ones you don't. Otherwise you'll have serious problems. It's like an electricity socket extender: Even though you can connect 4 devices to one socket, it doesn't mean you can connect 4 electric heaters, or you'll overload the grid.