Search engines such as Google and Yahoo! may update their algorithm dozens of times per month. When you see rapid changes in your rankings, it is usually due to an algorithmic shift, a Search index update, or something else outside of your control. SEO is a marathon, not a sprint, and some of the effects take a while to kick in.
Usually, if you change something on a page, it is not reflected in the search results that same day. Linkage data also may take a while to have an effect on search relevancy as Search Engines need to find the new links before they can evaluate them, and some search algorithms may trust links more as the links age.
The key to SEO is to remember that rankings are always changing, but the more you build legitimate signals of trust and quality, the more often you will come out on top.
Relevancy Wins Distribution!
The more times a search leads to desired content, the more likely a person is to use that search engine again. If a search engine works well, a person does not just come back, they also tell their friends about it, and they may even download the associated toolbar. The goal of all Major Search Engines is to be relevant. If they are not, they will fade (as many already have).
Real-Time versus Prior-to-Query Calculations
In most major search engines, a portion of the relevancy calculations are stored ahead of time. Some of them are calculated in real time.
Some things that are computationally expensive and slow processes, such as calculating overall inter-connectivity (Google calls this PageRank), are done ahead of time.
Many search engines have different data centers, and when updates occur, they roll from one data center to the next. Data centers are placed throughout the world to minimize network lag time. Assuming it is not overloaded or down for maintenance, you will usually get search results from the data centers nearest you. If those data centers are down or if they are experiencing heavy load, your search query might be routed to a different data center.