I know ive asked this before, but...

This seems to happen a lot I've found. No two sanding sheets are the same even though they are marked as the same grit. I've bought 400 grit that feels like 600 grit and 600 grit that feels like 400 grit. I think the only solution is to find a supply that feels right and grab a bunch of it.

Both the 800 grit you bought from Home depot and auto zone feels like 600 grit? Or is it only the one from Home depot? There's a difference between a sandpaper for general or wood surfaces and one specifically for auto paint finish. But bstrump is right, you just have to "feel" what's right and go with it. if it does not feell right then go to the next higher or lower grit as necessary.

Sandpaper brands are important. Find one you like and trust and stick with it.

I've found the ones put out by 3M are the best, especially for wet sanding.