I don't know exactly.
But I can say I prefer a speaker whose sound changes as little as possible, both vertically and horizontally, as I move off axis.
It sounds generally more "natural" to me and when there are obvious shifts in sound with minimal head movement, it tends to bug me.
I also sometimes sit upright, sometimes slouch, sometimes slide down the sofa in to almost lying position, so a speaker that sounds consistent vertically works better for me. (I remember having meadowlark speakers, first order/time coherent, which sounded nice in the sweet spot but there was an interference pattern between the drivers due, I believe, to the crossover design, such that raising my head, or standing up, produced a weird hollow-sounding suck out, and I didn't like noticing that).