Ask HN: Which feed count is more accurate - Feedburner or Wordpress?

I am having a hard time figuring out this rss subscription thing (from the admin point of view). The numbers fluctuate so much in feedburner, when I chose one specific day. How is it that the numbers on certain larger blogs, don't fluctuate so wildly? E.g. 37signals.com/svn

Also, why does Wordpress tell me a MUCH larger number (by a factor of at lest 3, sometimes even 5 or 10) for the last 7 days, than feedburner does?