The YouTubers can decide if they want to put ads on their videos, and whether they implement the Skip button. I believe the ones that you can't skip will earn them more money but you risk the chance of viewers just click away soon as they see an ad. Take IGN's videos, some of them are just news clips which can be 45 seconds long and there is a 30 seconds ad in front of it that you can't skip. You are gambling whether the viewer will willing to wait that long.
You can click back and click it again, sometimes it plays it without an ad, sometimes it plays a different one, but then it's a 30second ad, I have at times click back and forth like 5 times before I got the video with no ad but then by that point I must have wasted 25 seconds doing that….so I might as well wait.
/I think that's how it works….