STR:

1. Open https://www.newtech.fr/newtech/
2. Open the devtools' accessibility panel.
3. Resize the window so that the red text goes on top of the red part of the background image.
4. Inspect the red title text with the accessibility panel.

=> Notice that the tool tells that the contrast is good, while it's pretty clear it isn't.
I believe it works by taking the darkest and the lightest pixels only, but we clearly see in this example that it's incorrect.

Right now the way we determine contrast range is by taking background colour values that have lowest and highest luminance. As seen here this approach sometimes fails when there's an intermediate background colour value that might have a problematic contrast compared to the font colour.