Confused here in trying to learn the feel of threads and needle sizes. I think I see what may have increased this confusion.

I've been practicing with two Coats threads. They are both fresh, both US made, and both for machine sewing.

T26 Nylon "Upholstery thread"
T67 Polyester "Outdoor thread"

Tex. Weight in grams of 1000 meters of thread. If 1,000 meters weighs 25 grams, it is a tex 25. Larger tex numbers are heavier threads.
Here's the thing: The T26 thread looks distinctly MUCH HEAVIER than the T67. Makes it hard to develop intuitions, when it "ought to be" the reverse.

What accounts for this?
Are the differences in densities of these thread so great? Mislabeling?