Your system has a special receiver that allows it to receive digital broadcasts in addition to analog broadcasts.
HD1 signifies the main programming status and is available in both analog and digital broadcasts. Other multicast stations are only available digitally and could contain new or different content.
When the system first receives an HD1 station, it plays the station in the analog version until it verifies the station is an HD Radio station. Then it shifts to the digital version.
There is an audio mute delay when switching to an HD2 or HD3 station because the system has to reacquire and decode the digital signal.