Website not using retina sizes when available

I’m having a really hard time figuring out how my site is determining which photo sizes to use.

For example, on this page — https://theslowroad.org/back-to-the-middle-ages/ — on a full-width screen,
the first image (town on a hill) should be using the “wide” 1300px retina version, the second image (castle) should be using the “full-column” 806 px retina version, the third picture (flower on building) should be using the “medium” 393x590px retina version.

Instead, on my 1486px Macbook screen, the first image seems to use the 856 px retina version, the second image (castle) uses the 1300 px non-retina version, the third picture (flower on building) is using the 650×975 px non-retina version.

The retina 2x plugins say that all the sizes exist.

I’m so confused that I feel like I’m losing my mind. Please help!