While I can appreciate that reading about a topic I'm familiar with is more comfortable, as others have said, you should never use outside knowledge on a passage. Everything you need to do well on the CARS is in the passage and dependent on your reading and reasoning abilities. Is there really a huge difference in your scores?
1) Is there a vocabulary issue here? Maybe some words associated with particular topics are less familiar to you than others? For instance, if it's a political topic, maybe you aren't as familiar with words like aristocracy, plutocracy, kakistocracy...etc
2) You might be psyching yourself out. When you hit a political science passage, you start to think "Oh I never do well at these sorts of passages..." If you can stop that sort of thinking I think it will be really useful.
3) Some companies (shameless self-plug...) label their passages with the passage topic, so you can study with specific topic types. It might be useful for you to give a little extra attention to political science/government/economic passages. Knowing which passages are which helps you with this.
4) I'd recommend spending a little extra time reviewing the problem topic passages after you take them, in particular, focusing on the passage itself. We go through how to do this is in our free CARS guide here on SDN:
Day 26 – How to Review a CARS Passage. I think if you work through these sorts of passages slowly, and practice summarizing and re-reading them after you take them, you'll both pick up any vocabulary you're missing as well as become more comfortable with these sorts of passages.