Have you been logged into an account when searching google and then logged out other times, I've read that this can sometimes affect rankings.
If it's a keyword / keyphrase that is popular and has alot of SEO people gunning after it then it's likely BH experts will get their sites in the top three for short periods of time and then google will detect and rectify the problem.
This is just guess work as google does what it wants and we can't tell why really.
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