The biggest surprise of these is the community wiki category, the white text is much easier to read for me (so the automated choice feels correct) but the contrast checker suggests black text is better. There is likely some level of taste/preference here like you suggested.
The tricky part is that we are looking specifically at contrast, whereas I think color brightness also influences what feels easier to read too.
Generally speaking I think this works quite well for the majority of color ranges. For example, if you choose a dark color it will suggest white text, for light colors it will suggest black text. There are some colors that are in between that could go either way, and in those cases the user can pick what feels right from the edit category page.