One, the spectra support lines aren't ZERO stretch. They're LOW stretch. When my ULB was new, I got quite a lot of stretch out of the cords. Even after they've been "broken in," I still see more stretch (per foot, of course) than I do with the Spyderline I use from APS.
Two, my HH ridgeline stretches significantly. I always assumed it was spectra, but now that I think about it, I have doubts. Anyway, if the ridgeline stretches, that explains changing the sag by tightening the suspension. I've actually experienced this effect myself. It's not nearly as pronounced as a non-ridgeline hammock or my couple of attempts at using a stretchy ridgeline (550 cord), but it's definitely noticeable.
I never measured the stock tree huggers to see if they stretched or not - mostly because I chucked the whole suspension as soon as I saw something better. I will say that I never noticed any significant stretching in that area.