The jack is a stereo jack but the output on both channels is the same, i.e. mono.
If you plug in stereo headphones you will hear the sound in the middle of your head as the output from each transducer is identical (or at least as identical as the drivers allow).
Ed's right. I think the reason they used a stereo jack is because it's impossible to buy a mono earphone today. Everything is stereo and a stereo plug in a mono jack can cause problems.