Here's a little Grainger article about sandpaper and it's qualities.
Sandpaper Grit Chart & Guide - Grainger
Here is their statement at the bottom regarding qualities of sandpaper. Clearly, if you buy enough sandpaper, which I have, you will notice a difference in brand names. I've clearly noticed differences in quality of paper but it was never so bad I had to worry about. That being said, although I have an ok stock of sandpaper I will generally find one I like to work with and use it - but I never put any serious thought into why they were different, I just figured one was more cheaply made but the quote below pretty much answers any of the questions I would've had
What's the Difference Between Sandpaper Grit and Sandpaper Grade?
Grit size refers to the specific size of the abrasive grain (36, 60, 120, etc.), while grade is a broader term, with each grade including a range of grits. For example, medium grade includes grit sizes between 80 and 150.