Drywall – Latex Matches Perfectly Twice…Then Suddenly Darker

This may be an amateur question, but I am racking my brain to figure out what is happening. Appreciate any advice.

I am doing touchups to basic drywall with a flat latex, very light gray. I have the original paint that was used just a couple months ago, and so it has matched perfectly. One section I painted had some raised spackle though, so I sanded the paint itself and the spackle back down and painted it again. The paint matched perfectly again, but I hadn't painted as far to the edges as I should have, so a couple days later I went over it real quick with another coat.

This time, after matching perfectly twice, the same paint was a whole shade darker. It stood out like a sore thumb. The exact same paint, on the same spot, with the same roller, just two days after it had matched just fine. No dramatic temperature or humidity changes.

I've sanded the last paint coat off again, and the layer underneath is much closer to the right color, but now I don't know what to do. I've dabbed a few spots, and it seems like the paint is still drying too dark.

Is there some property of drywall that dictates that you can only ever paint it twice before paint starts drying darker? Or does one have to wait for longer than two days just to do another matching coat of a flat latex? This stuff dries in like 5 minutes so that doesn't seem right. I just can't figure out the physics of it, and I really want to be able to make it look right again.

I wish I had pictures of the whole process but I only have them as it exists now – I have sanded back down, you can still see the color and texture difference. It seems clear that just painting it again will still yield the darker color. I did a little test with primer too, but that didn't seem to change much. How can this suddenly change from matching perfectly to not matching at all?

Thanks!

UPDATE: After sanding nearly to dry wall, I tried a dab just to the right of the hole, you can see it fairly clearly. I think it matches the surrounding paint pretty darn close. Thoughts?