Just because no one has explained it....ENO Slap Straps and ENO Slap Strap Pro's are made of nylon webbing and are well known for lots of stretching. The ENO Atlas Straps are made of "Polyfilament Webbing" (probably polyester) and don't have the stretching problem.

Nylon webbing stretches an undesirable amount...polyester and polypropylene have minimal stretch. Polyester webbing is most commonly recommended for tree straps and suspensions.