We pre-sell advertising and then use 3rd party services to fill our remaining ad inventory. For these 3rd party services, we specifically block anything political or not intended for a general ages audience. There are settings we can use to specifically block them, and we pretty much block all of the filterable categories.
That being said, once in a while an ad can fall through the filter if someone administrating these services doesn't flag the ad properly. That's probably what happened if you saw a political ad. If it's coming up repeatedly for you, please screencap it and send me a link to the image via PM. We can find the ad in the appropriate system and report it for not being flagged properly.