I have no idea!
Makes sense. I also don't think google can distinguish between the different sites.. if it works like that for those subdomains, I'd bet it works that way for all the others too.
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In reverse... a subdomain is no different than a folder today (in Google anyway) so wordpress could be structured like wordpress.com/my_webspam/ and it only effect those issues on those particular pages.
disavow is exlcusively a Manual Review tool... which suggests Google Webspam Team controls how they devalue your domain... a page, a folder, and subdomain or the complete domain.