Why are ads in iTunes Radio so inconsistent? Sometimes they occur after every song, other times you can listen for extended period of time without hearing any? I don't want to purchase iTunes Match, I just wish the ads were more consistent.

I understand that in order to listen to iTunes Radio ad-free, you must subscribe to iTunes Match. I don't want to pay for this service, so I am fine with the occasional ad. However, the frequency of ads seems to be really inconsistent. There are times when an ad plays after each song and other times that you can listen for an hour with none. Any ideas for why this happens or suggestions for how to avoid this problem?

Thanks!