I do not think this is a ranking anomaly, I just think the results are being personalized despite the common belief associated with Incognito mode. Check out the link below which will take you to the Google Chrome Forum post 'Does Incognito Mode Remove Personalized Search Results'. The top answer provides a great explanation as to why these results are not always as unpersonalized as one might think.
https://support.google.com/chrome/forum/AAAAP1KN0B0Zaq9rFwdLWg/?hl=en
"When using Incognito Mode, you’re most definitely not anonymous; you are exactly as identifiable, trackable, followable, and recordable as a Normal Chrome window. All browser info, system, plugins, IP connection, location details, etc. are provided exactly the same in Incognito, and the websites and webservices you go to are provided all the same info as a Normal window. Incognito effectively provides a very quick way to start an empty session; but if you keep using Incognito without ever closing the last Incognito window, you are just as identifiable and trackable as using a Normal window, and you gain nothing by using Incognito. (If you want true web anonymity, you need to use proxy servers or proxy services.)"
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