For a number of different reasons which likely include mixing (http://en.bitcoin.it/wiki/Mixing_service)  the number of bitcoins sent in a day appear to be a much larger number than what likely is truly being transferred.   Other payment systems know to the penny how much was transferred in and out and they use that metric in measuring growth, for instance.

With the total coins sent being a number that so badly misrepresents the total transferred those wanting statistics on Bitcoin had to come up with an alternate method.  The BitcoinDays Destroyed metric was devised to help show how the currency is used and is about the closest possible method to the total transfers metric that competing services report.

Essentially the BitcoinDays Destroyed increases when coins that had previously not moved start moving.  The older they are, the more "days destroyed" when they are eventually spent.  Re-spending those shortly after will have a tiny amount of "days destroyed" because they weren't idle for long.

 - http://en.bitcoin.it/wiki/Bitcoin_Days_Destroyed