Maybe it’s because you cook with gas?
I have the same cookware, and it has the same perfectly-straight unanodized line on the inside at the typical water line. There’s no way it’s from scrubbing, as there’s no way I scrub the pot that straight. The pot is typically used for pasta, which we typically salt the water. Maybe the salt has an effect?
One interesting thing I noticed is that if I boil some aluminum foil, the unanodized part gets dark again. It lasts for a little while before wearing off. I’m not sure if it’s anodizing it or if it’s something else. The reason I boiled aluminum foil is I was making some eggs and didn’t want them to sit on the hot bottom and bounce around from the steam bubbles. I set the eggs on some crumpled up foil and noticed the darkening of the shiny part of the pot afterwards. (ETA, maybe it was the combination of eggs and aluminum foil. I never boiled just aluminum foil to see what happens.)