This is a bug, which we're looking to address. As heath mentioned below, its supposed to be that you only ever see an ad every 300 seconds at most, and that you don't see one on videos under 30 seconds. YouTube/Vimeo embeds are separate from the second rule as we have no way of telling how long they are from the embed code.
Well that isn't nice! We have a business to run here... how do you propose that we make money to keep NS running with everyone using AdBlock?
Would it be better if we just snuck all the advertising directly into the content so all of it was paid for?
Would you be willing to pay a subscription?
Just caught a bug that came with a recent update. As mentioned above, YouTube and Vimeo we can't tell how long they are server side, but the NS videos should be fine.