I don't think you can mandate the frequency or volume, as it's likely just due to unsold inventory. Even network TV & cable has it on occasion, they just fill it with 'free' or 'added value' spots, or PSA's. But I think it just means the ad buyers are only willing to pay a limited amount per day, and the ad servers spread it out to not go over.
As for the Roku ads, I'll check this weekend but I thought I saw something in T&C somewhere saying something about if you use your own ad server you authorize Roku to fill 20% of inventory or something. Don't quote me on that, but I thought I read that somewhere.