Makes perfect sense. Leaving the top 10 or 50 static for extended periods wouldn't make much sense. First, that would leave no way of using click data to correct poor choices that the rest of the ranking algorithm has made. Second, that would leave no way to respond quickly to changes in user needs.
A page that contains the words "Brittney Spears" and "giraffe" might not deserve to rank highly for "Brittney Spears" today. But if she gets bitten by a giraffe tonight, that could change -- much faster than can be handled by the crawl-index-rank-export_to_datacenter cycle.
It's not as though Google has to constantly be making gigantic changes in the top 50. They merely have to conduct sampling tests periodically to see if it reveals that a lower-ranking listing is currently more relevant than those on page 1 or 2.