While Feedburner statistics are pretty accurate some readers might end up not using the Feedburner feed. If you start using Feedburner when your blog already has some subscribers, for instance, they will keep using the old feed and thus will not be counted on your statistics.
Alternatively even if you have the Feedburner feed on your links some people or RSS readers might subscribe to secondary feeds like the RSS 2.0 or Atom feeds provided by WordPress directly. In order to redirect all feed requests to Feedburner you can use a plugin called Feedburner Feed Replacement.
The plugin will not increase the number of RSS subscribers, but it will forward all the subscribers to Feedburner, making sure that they appear on the feed count and giving you a more clear picture of that number.
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