Spammers sometimes create an entire message composed of nothing but script, such as Javascript. Before the message is loaded, there are no words that the statistical filter would be able to identify. When the message is loaded, the text is displayed as normal, instead of the script.
The HTML parser ignores the script tag in messages. Therefore, if you want messages with scripting to be identified as spam, select Script Tag under HTML Feature Filtering. IMail will identify all messages containing such a tag as spam.