That is because Yahoo polls (checks for new content) only once an hour. You will not get the information instantly. RSS is a "pull" technology rather than a "push" technology (like email).
If too many people are polling a feed all at the same time, or too often, it can cause real problems for whoever is running the servers. Anything more than about once an hour is too much. The web-based centralized aggregators like yahoo, bloglines, msn, googlereader, etc, usually poll about once an hour but they also don't poll every single feed at exactly the same time, otherwise there would be this huge load coming at the top of the hour from everyone. (This is a problem with many desktop aggregators, because their default setting is to poll at the top of the hour, so sites get this big rush all at once.)
In normal web surfing, it's true that at some sites you may actually sit there and hit the refresh key every minute or two. But very, very rarely. With RSS, a machine is essentially doing that to every single site that you have as a feed, whether you ever go there or not, whether you truly need the information or not. This is needlessly wasteful. No one needs to keep pounding 50, or 100, or 200 sites checking for updates every two minutes. No one can possibly visit all those sites every two minutes. Once an hour is plenty.
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