Even though most of us have eliminated sources of stretch in our systems (polyester or Amsteel for suspension lines, polyester for tree straps etc) we are still left with the nylon hammock bodys.
This is just a hypothesis on my part, but I think that the hammock stretches initially when first loaded, but as the night progresses moisture from insensible perspiration and humidity from the air probably infuse the nylon threads and cause the fabric to "relax" a little more. This would add tension to any UQ, even one that was very well fitted to the hammock body.
So far I have not tested my hypothesis. Probably not going to, as I am happy with shock cord suspension!