Since this is in the AdSense forum, I will not comment on search engine results except when they have an effect on earnings as that is what you are asking about.
I am not very well versed on the various terms or keywords that earn 'big bucks' for Google AdSense publishers, but in my experience over almost 10 years as a publisher, I can certainly state that the effectiveness of advertisements and valid CTR/CPM are almost unpredictable, to a degree and over a period of time.
Less traffic can mean more clicks and vice-versa. Depending on your traffic (which coincides with SERPs), 2000 impressions and 20000 impressions could yield the very same overall income with the very same (or similar) content and ads; Now-a-days, it seems that when you have very well-targeted traffic, less is more and more is less in terms of traffic:income.
In terms of sites with little-to-no updates, I would have to say that it is more of a roller-coaster ride which is more unpredictable. As I said above, there is very little predictability regarding traffic and earnings in my own experience and the traffic vs. CPM ratio can be interchangeable. Seeing weeks or even months with surprisingly high CPM are to not be totally unexpected if you really provide content which is complimented by useful ads.
Focusing on the short term is never a good idea.