Because engine paint wasn't important enough for Chrysler to spend a lot of money on. There was no reason why the engine in this car should have to be exactly the same colour as the engine in that car; the paint was purchased on a low-bid basis and one of the ways the paint companies saved money was by going ahead and shipping paint that wasn't quite the same colour as the previous batch. It's not like Chrysler was going to stop the production line because the engine paint colour changed a little.
Me, when I was restoring a 1965 Canadian-built slant-6, I found that the Mopar Performance red wasn't even slightly close to correct, but "International Harvester Red" spray paint made by Martin-Senour and bought at NAPA was an ideal, exact match. It was a deeper red than this what you show us in the photos.