It's not a perfect system because there's no way for any filter to know exactly what the content of a site is or whether it's an "adult" site or not. This is from Apple's site explaining the filtering:
When "Allow unrestricted access to websites" is selected, the Internet content filter logs websites that the account visits but does not restrict Web browsing. Visited websites are still logged and can be examined in the Logs tab of Parental Controls preferences.
When "Try to limit access to adult websites automatically" is selected, the Internet content filter does its best to block websites with inappropriate content. To do this, the Internet content filter uses the same technology that the Mail application uses to identify "junk" mail. The Internet content filter can identify, with a high degree of accuracy, whether a Web page is safe or not by examining various properties of the website including text and structure.
Additionally, the Internet content filter will block a website if the website identifies itself as adult-oriented using RTA (http://www.rtalabel.org/) or SafeSurf (http://www.safesurf.com/ssplan.htm) rating systems, as well as forcing "safe" searches with some search engines.
In this mode, the Internet content filter logs all visited and blocked websites, and flags them as such in the Logs tab of Parental Controls preferences.
In certain situations, the automatic Internet content filter may mistakenly block a safe website or allow an adult-oriented website. For example, if the website uses an uncommon language or if there is very little text on the page. These websites can be identified in the Log tab of the Parental Controls preference pane and added to the "Always Allow" or the "Never Allow" lists. These lists can also be accessed by clicking the Customize… button in the Content tab of Parental Controls preferences. Websites that are mistakenly blocked can also be allowed by clicking the "Allow…" button on the blocked web page and authenticating as an administrator user.
So there's no way you're going to be able to do perfect filtering.