My friend had this extra Y prong adapter for airlines that converts the single stereo headphone jack to the two prongs for airlines. This adapter has two wires coming out at the end to plug into the two prongs in the seat with a single wire for my headphone. Will this type work on Delta as the package said Y Prong adapter for airline.

I was wondering on the 737-800 when I tried plugging in my singe prong headphone I got stereo sound but the volume control was funny where I pushed it up and down and it went from soft to loud. My question is why would one use the two prong adapter when I got stereo sound from plugging in my headphone to the jack? I wonder why Delta does not use a single prong. I got stereo sound on my headphone where the sound came form both ears. On yet another flight I tried plugging in my headphone this way and only got mono sound on the 737-800 so I used an old adapter which I lost and it worked. Any ideas as to why this happened?

Thanks