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Then there are some other keywords and phrases (in certain "industries") that change every 2 to 4 weeks.
Not that I mind since I've benefitted from time to time, but I wonder why certain "for-profit industries" are so volatile while others seem so stable.
1. In the competitive area many sites may have very close rank. Therefore even a small change in rank may dramatically change SERP. If there are many sites the changes in the rank are frequent. It will not happen however if there is a clear leader with the rank much greater than other.
2. In competitive area Google may rotate the results to observe the user behavior.