Dumb question but how would they know the content is different unless they're also crawling incognito and comparing the results?
Presumably, the incognito crawlers are only used on sites that have already granted the regular crawler access. That's content that ends up in their index which they want to vet.
https://www.theguardian.com/technology/2020/feb/03/berlin-ar...
Also, I would've sworn that happened circa 2015 and not 2020. The passing of time for the last few years has such a muddled feeling.