does anyone know why the sky130 hd tap cells have a seemingly-unused psdm region next to the tapped nsdm region in the n-well, and equivalently an unused nsdm next to the psdm in the pwell? (here is n well)

continuity. standard cells are often placed in rows, with the alternating rows being flipped on the X-axis. The seemingly unused nsdm/psdm regions overlap. It is possible that 2 tap cells might almost align vertically. If the seemingly unused nsdm/psdm regions were missing, there could be spacing errors between the nsdm/psdm in the normal standard cells.

but you can achieve the continuity if you just extend the tapped nsdm/psdm region, you don't need the alternate to be there?