Specifying very small font size, having white colored text on a white background, and CSS styles that are hidden are all techniques used by spammers to hide data. Typically, their goal is to make the reader look at a URL or an image. Having a very short email only containing a URL is often caught by content filtering. Spammers know that and therefore, try to confuse filters by adding irrelevant content that is invisible.

Try running the report for Custom Filters (under Reports). That will give you a summary of messages that are caught by every rule, allowing you to adjust their scores if required.