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Usually your site tends to bounce around a lot when it's new, as Google tries to figure out where you belong in your SERPs. If your site has been around for 10 years, then that's probably not the case. Personalization is the first culprit I would look to, and if it wasn't that then it could just be Google testing a new algo tweak.

Yes, try looking at the server response. Screaming Frog will tell you this.