Original: Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices will transmit and receive at USB 3.0 data rates regardless of USB 2.0 or earlier devices connected to that host.
Simplified: On an USB 2.0 controller, standards can be mixed. Devices connected to a USB 1.1 hub will only go as fast as USB 1.1 rates.