I wonder if it's because of the font-weight being decreased. If I disable the `font-weight` rule in Firefox's Inspector the text gets noticeably darker, but the contrast score doesn't change. Could be a bad interaction with anti-aliasing thin text that the contrast checker isn't able to pick up.

I'd say it looks pretty readable on android although I still wouldn't describe it as good. I wouldn't say I feel encouraged to squint. But possibly different antialiasing explains it.