I'll also add, if you zoom in and look at the top of the card against the walls behind, there's patch right of centre where the colours are identical. It's all just about light.

No, it does not appear to be the same colour because all colour is just reflected light, and these surfaces reflect light differently. They face in completely different directions and you are holding the sample card in a third completely different direction.
If you used paint from the same pot/tin to fill the gap it is clear the same paint, it just appears different colours because of the angle of the light source. The textured paint exaggerates the effect, but do you genuinely think the paint is changing colour when it comes into contact with the two different surfaces?

Further test - cut the sample card in two. Tape/paste a piece against each wall right into the gap so they touch.

It'll look much better when you paint into the gap. There's a combo of effects here:

1. light striking the right hand wall but leaving the left in shadow creates very different tones.
2. The dark brown and bright white when placed next to a mid tone (your new grey paint) will each make the mid tone look very different - the dark brown makes it seem brighter than it actually is, and the white makes it seem darker than it actually is. Placing those effects side by side creates an even stronger contrast.

Switch from pods to basic powder.
I spent over a year trying to solve the same issue - repeated cleaning, cycles run with vinegar and baking soda, dedicated washing machine cleaners, hot cycles - nothing worked for more than a couple of weeks.
Someone recommended stopping using pods and switching to older tablets or boxed powder and thought it was worth a shot.
We now spend something like half the money on detergent, our clothes come out cleaner, stains shift better, everything smells fresher and the machine has been spotless and odour free ever since.

Pods are a scam!

Anyone else see this and hear the speeder bikes on Endor?