This thread is darned long and I don't know if I can wade through to remember if I made a comment about this.
When I first got my Lifeline chest expanders and long tubing perhaps ten years ago, I was surprised when their claim of 'Xlbs at 2x stretch' felt a lot lighter than weights of the same poundage. I did my own makeshift testing by hooking one end of long tubing to a dumbbell and pulled upwards until it reached twice its length. I used different weights until I found the right weight where it lifted the dumbbell off the floor at 2x stretch.
The purple cable tested at about half of the stated pull weight. The red cable pulled at about 2/3rds the stated pull weight. While I found this disappointing, I never asked Lifeline about this, and just went about my training with this in mind.