It is probably based on the wording. I get things show up in spam one day from source I get email all the time. It is often the content, it says something common to spam.
As @donoreo said. It's an automated program that checks content but also looks at reports from other GMail users who flag it as spam.
I accidentally flagged email from one of our server's weekly security report as spam and it took a few weeks of me clicking on "not spam" before GMail settled in to passing it through.