Headset audio out-of-phase

Somebody on one of these forums asked about how a headset with mic has its wires connected.
I found a stereo headset with mic.

I plugged it into my pc speaker headphones jack to hear how awful is its sound and played a song.
Huh? What? Where is MY song? It is playing another song.

I guess its wiring caused its earphones to be connected in series so the mono was cancelled and it played only the difference between channels. The singer and bass were almost completely gone. The background music was very clear.

It is wired like this:

Years ago I mixed stereo to mono (out of phase) to remove the vocals. Often worked.

I played a place once where the house recorded the band using good equipment including two overhead expensive mics.
The bass was almost completely canceled. I also recorded the band with a mini-disk.
The bass was fine.