Why are some SERPS volatile & others not?

volaility vs. stability

[sore66]

I notice some of the keywords I track and target on Google serps NEVER change results. The same organic and not-for-profit sites and the same image results remain month after month.

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.

[Vadim]

I believe that there are at least two reasons.

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.

Vadim