Why might only some papers from a conference be included in Google Scholar? Why some publications are not included in Google Scholar, while some are, for the same conference?

For example, there are a number of papers on Black Hat:

https://www.blackhat.com/docs/us-15/materials/us-15-Arnaboldi-Abusing-XSLT-For-Practical-Attacks-wp.pdf

https://www.blackhat.com/docs/us-15/materials/us-15-Evenchick-Breaking-Access-Controls-With-BLEKey-wp.pdf

But when you search for them, like "Abusing XSLT for Practical Attacks" on Google Scholar, there is no result.
They are papers in 2015, it has been one year since then.

Also, they meet the Google Scholar's inclusion requirement:
https://scholar.google.com/intl/en/scholar/inclusion.html

However, for some other papers in the same conference (Black Hat USA 2015), they are included in Google Scholar.

So what are the real standards for publications to be included and searchable in Google Scholar?
Doing a little bit of poking around, I found most articles on BlackHat 2015 not listed in Google Scholar, and all of the articles that I did find listed were cited by other papers.  

It seems to me then likely that this is among the "grey area" sites that Google Scholar is not indexing (BlackHat's status as an academic conference is murky), and thus that any material that you find is being added to the Google Scholar index through other routes.