Double conducturs on 3.5 3-pin jack?

I am looking at the cable for the Bose quiet comfort 35 and there is something I don’t get.

It’s a 3-pin 3.5 jack to 3-pin 2.5, so I expected to see 3 cables.
Left, right and ground, but… I found 2 left, 2 right, a mic and ground??

What gives? The headphone has a mic, but I thought you needed a 4-pin for that? In other words the mic would only work via bluetooh.
Never mind the double channel cables.

It ends in the 2.5, it doesn’t run all the way through to the other side.
So what are the secondary channels for?