CEO of Increase Media Agency - a Digital Marketing Agency with 22 team members that helps top companies from 30+ industries grow on social media and find their full potential online. ✔️ Facebook Advertising Blueprint Certified Let’s talk!
First off, Facebook's algorithm is auction-based, so there's no literal way to tell why something costs less than something else, even though they're completely identical, which is not your case.
In your case, If your CTR is higher on one of the images, that shows facebook that your audience is engaging so they're rewarding you by giving you a smaller cost per click. So that's why.