Just like a girth hitch can vary in strength depending on how much bend in the rope is created by where the intersecting rope surfaces are, so does the timber hitch.

So one would end up with a range of strengths for the same rope, hitch and anchor.

The tensionless hitch is a better way and has the bonus of being able to be released under tension. Just unclip the biner and unwind. (This can be useful in a rappel situation where someone gets their clothes or hair stuck in the rappel device and is also on a backup belay. Hold the person on the belay rope, give a few feet of rappel line, then reclip the rappel anchor. Then the person can clear their system, belayer release the belay, and continue with the rappel. Much like using a Munter/Mule system on the rappel anchor for similar screwup solutions. Cheaper than cutting the rappel rope free of the anchor and safer than trying to use a knife to cut clothes or hair away from a tensioned rope.)

The timber hitch holds better when dragging timber (imagine that) as it cinches down as the log turns, rather than unwinding, with less rope needed to make the hitch.