USB 3.1 in general is a mess. The spec allows for Alternate Modes, where the port is hardwired on the Motherboard to be able to be switched to deliver DisplayPort, HDMI, MHL, Thunderbolt, and damn near anything else you can figure out how to mux over the port's pins and cheaply demux on the other end of the cable. The best part is no one will actually list what Alternate Modes their laptop or motherboard supports. So if I want a laptop that supports, DisplayPort Alternate Mode with dual monitors, I'm opening support tickets with manufacturers asking "Does your USB-C port run this?"
Apple and Intel simply followed along what with what was already a mess.