This is likely because Google had such a large userbase that they were able to update more people's feeds at the same time than a lot of other places like newsblur and feedly can right now.
I also imagine it's because google was requesting the feeds more often than others, or leveraging the feedburner system to know when they were updated and other such things.
Google Reader implemented PuSH:Pubsubhubbub (worst protocol name ever!). Most blog platforms ping using PuSH, so the update needs no polling and is thus almost instantaneous.
I also imagine it's because google was requesting the feeds more often than others, or leveraging the feedburner system to know when they were updated and other such things.