Apple obviously installed a slower/cheaper SSD in the MacPro as the PCIe 2.0x4 connection couldn't handle a faster one.
The later MacMini/iMac/MacPro have at least PCIe 3.0x4 connection (newest MacPro also has 3.0x8 and 3.0x16 slots), so it can use a lot faster SSD (to make the dog that is APFS/Catalina feel snappier).