Probably based on keywords related to ads in their system just as with any other website using adsense.
You can use Analytics within YouTube to see which of your videos have most monetized content, and which videos have highest paying ads, so you can try customize new videos towards the same topics and keywords, as well as focus your time on best paying videos.
Sometimes a video may have for example 10 000 views, but only 5 000 monetized views, and earnings of $100 - while other times a video with 10 000 views may have 9 000 monetized views but an earning of only $50. Thus, it would obviously be more of your interest to increase popularity and views of video 1 with $100 earnings yet less monetized views, instead of the video 2 with more monetized views but an earning of only $50 for same amount total views. All those statistics are available inside YouTube via the Analytics (available in your channel page when logged in)
Obviously, some topics will give you more adsense earnings than others, so try around, test a few different niches, and see which ones are most rewarding, then focus on those.
As a more direct example, I have one video now with 75 000 views and only $190 in earnings, and another video with only 15 000 views and $240 in earnings. Obviously I will focus on getting more views on the higher paying video with only 15 000 views, and try to make it go virals.
Always check what videos gives you most $ per view - then blast that video out to the public to get more views and cash. Use the link sites I recommended, and use forums. Use the tools I recommended. It WILL boost your income!
As another example, I put my Tube Toolbox into action for a few hours yesterday, added around 400 contacts in youtube with it, and I also posted a couple videos on the link sites (Digg and Stumbleupon) along with a favorite high-traffic Forum (where my video got pinned, which they get all the time and thus exposed to LARGE amounts of viewers). Doing that have already made me make $100 more today than my daily average, and the day just started here
Would like to try out this method but how to set up your adsense account with a youtube account if they have different emails? My main account on youtube was suspended...
Please from now, read the new updated thread and reply there:
I will request this old outdated thread here to be locked.