And without wishing to add another factor to the calculation, here is one! I don’t know about our North American friends, but in the UK it’s quite common when booking seats to request those with extra leg room. In fact I’ve just done that for a Jet2 flight (737-800) to Madeira in a couple of months. So, IF my assumption is correct that these seats on the Alaska flight had extra leg room, to me it would be even more surprising that they just happened to be unoccupied.