What causes Google to misread these sizes? Using Chrome’s Inspect Element feature, the first is scaled down to 600x750, but as this changes when I move the Inspect Element bar up (resizing the image) I can't find the native size of the second image.
The second image's native/true resolution is 768x960, which does match what Google image search said.
It is indeed strange that the first image's native/true resolution is 600x750, but Google image search reports as 800x1000. At first, I suspected Google was reading the images' metadata tags, but I looked at the EXIF and IPTC tags, and they were correctly showing 600x750.
My best guess is that originally, the image was 800x1000, Google's search engine cached it. Then the user resized it and reuploaded it again.