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I have noticed the same since last Februray, especially after Bourbon update.
I guess the main reason is that the datacenters are in everfluxin (the state of everchanging unstable serps due to continues updating process including addition of new pages, backlink update, algo update etc.). Therfore its impossible sometimes to track the exact positions of your pages on the serps within the same day or sometimes even within few hours!
I have literally seen keywords change from minute to minute.
The interent results have really changed over the last couple of years. There are so many websites with in some case millions of sites competing for the same keyword.
Results change quickly, everflux effect.