Why is this text, that fails WCAG accessibility guidelines, subjectively easier to read than text that passes?

1. Why just following the guidelines may not provide 100% accessibility compliance?
2. What are the WCAG 2.1 guidelines?
3. Is white text on red background accessible?

Why just following the guidelines may not provide 100% accessibility compliance?

Even after you follow all of those best practices, why do we say, “don't claim 100% conformance with WCAG?” Here's why: when it comes to some of the WCAG success criteria, there are gray areas where even experts disagree, and you can't guarantee that there won't be usability issues across an audience with varied means ...

What are the WCAG 2.1 guidelines?

Web Content Accessibility Guidelines (WCAG) 2.1 defines how to make Web content more accessible to people with disabilities. Accessibility involves a wide range of disabilities, including visual, auditory, physical, speech, cognitive, language, learning, and neurological disabilities.

Is white text on red background accessible?

Common colour combinations such as white on red appear to be inaccessible when they are not, and other combinations such as blue on grey appear to be accessible when they are not.