@depglass I can shed some limited light on this.
I read quite a while ago that pilling, breathability, and other characteristics are affected by whether the fibers are woven or knitted.
Unfortunately I forget which causes what because of the variables in the fiber itself.
But I'll compare my soft fuzzy warm blankets to your poly fleece sheets.
One short nap blanket makes me sweaty in 10 minutes, one long nap blanket never does because it breathes. Both 100% polyester.
But these two never pill, while my original Fluffie, also 100% polyester, is a magnet for cotton-lint pills.
So it's the construction of the fiber, and then whether they knit or weave it. You're doing nothing wrong in laundering.