Colors changing when pasting within same document?

I'm having a hard time figuring out what exactly is happening here. At one point, I noticed some of the sprites I was working on were a slightly lighter shade than what was copied from the same document.

The pencil used was 100% opacity. The layers were all 100% opacity. Anti-Alias was disabled. I've ended up with a color index of 80+ while sampling from only 15 or so colors. I can't think of any reason why they'd be changing like this.

In this related example, these silhouettes of copied sprites were left on the background, like they were burned into it. A copy with a pink back is included to show the difference, since it's kinda hard to see. Does anyone have any idea what's causing this? I'm using Photoshop Elements 13.

[ moved to Photoshop Eements by moderator ]