-they are being caught and held in the spam filter
The vast and overwhelming majority of all so-called information on the internet is spam. Over 80% of all so-called comments submitted to .wordpress.com blogs every day are spam.
The worst of all spam containing malware and viruses is not sent to us. It's removed as it could bring down many blogs if opened.
On the spam that Akismet catches it takes only seconds to click "empty spam" and you don't even have to click "empty spam" if you don't want to. Akismet will maintain comments it has caught for 15 days from the time they were received, and then it will delete the comment automatically.
If you get spam that slips by Akismet then mark it as spam (do not delete it) and over time Akismet will learn it's spam.
There is no such thing as eliminating all spam and Akismet is handling your spam correctly. Please don't brood about this. For more information please take the time to read "5 things every blogger should know about spam" on this page Akismet: How it works http://akismet.com/how/