That all makes sense.

Now, back to my earlier iteration, I'd contend that the effect was intentional, since it's observed in all ribband sections of the tube. If it was unintentional or a mistake, it should be localized to one section.

I was hoping DocDrew would be able to provide a specific name for this feature.

Regards
Ken

I prefer wood to plastic, leather to nylon, waxed cotton to Gore-Tex, and split bamboo to graphite.