I am assuming it’s an EMU thing. You rarely see double DMUs off-peak (in central Scotland at least) but EMUs seem to remain in their formations all day.That seems to be an international trend. On the Vienna S-Bahn, we used to have lots of 6-to-3-car „weakenings“ off-peak and on the outer fringes of the network. That has virtually disappeared. High staff costs for the maneuvers required and threats to reliability are said to outweigh increased operating costs (which are probably quite a bit lower for electric traction than for diesel) and maintenance needed.
I used to be jealous of the 6-car 334s that would rock up to Haymarket mostly empty off peak while my Glasgow or Dunblane train would show up as a single Class 170 or a single Class 158.