There has probably been at least one time when you noticed that one of your pages used to rank for a certain search query, but then later dropped for some unexplained reason. Matt Cutts, in the latest Google Webmaster Help video, talks about why this might be the case.
Cutts responds to the following submitted question:
When we create a new landing page with quality content, Google ranks that page on the top 30-50 for targeted keywords. Then why does the rank get decreased for the next 2 to 3 weeks? If pages didn't have required quality, then why did it get ranked in the first week?
"That's a fun question because it sort of opens up how writing a search engine is kind of a complex task," says Cutts. "You're basically trying to make sure that you return the best quality result, but you also have to do that with limited information. For example, in the first minute after an earthquake, you might have different people, you know, saying different things. You know, ten minutes after an earthquake you have more information. An hour after an earthquake you have a lot more. With any event that has breaking news, it's the sort of thing where it can be hard to know, even if multiple people are all saying the same thing, and one person might be the original author, one might be using that RSS. It can be difficult to try to suss out where was this content appearing originally."
"And over time - over the course of hours, or days, or weeks - that gets easier," he continues. "But it can be harder over the course of just minutes or hours. So a lot of the times whenever you see something ranking for a while, we're taking our best guess, and then as more information becomes available, we incorporate that. And then eventually, typically, things settle down into a steady state. And then when there's a steady state, we're typically able to better guess about how relevant something is."
As Cutts goes on to note, Google finds that freshness is deserved for some queries, while evergreen content works better for others. In my experience, Google struggles with this a lot, but seems to give more weight to freshness more often than not. Of course, I'm typically writing about newsy topics, so that makes sense to some extent (though there are plenty of times in researching topics that freshness gets a little too much weight).