Not sure where the seatmap starts (there are no row numbers), but looking at that, is seems like an older layout; if the map starts at row 16 on a 77Y, the middle seat in question would be 20K.

Could it be that the person selecting that seat did so by phone and thought it was a nice aisle seat in PP on a Polarized plane???
Or maybe that middle-seat person had an equipment swap from a Polarized version?

(Maybe not the case of a 77Y, as the seatmap in E+ are slightly staggered on those.)