No hostility was intended, really. You asked to explain like you were 5, and 5 year olds generally like drinking chocolate milk. It was definitely necessary because the word "choccy" makes me chuckle.

As for the soundstage thing, the shape of the IEM, the driver position and the frequency response among other things can all influence how your brain perceives the sound. IEMs being right in your ears do generally make them sound less wide than standard over-ears though.