Thanks for the info. The adsense team replied to my Email, though the answer was vague, nevertheless my mind is at ease for the time being. Here is the answer in detail:
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If overlay ads and non-skippable ads formats are not appear next to your videos. Then it means that you are not eligible to choose these ads format type yet. Please do not worry since those ads formats will not effect any of your revenue.
As YouTube welcomes more monetized videos, we want to be as transparent as possible about the process, while helping you understand that revenue will fluctuate.
Advertising is also flighted. Different ad formats will run on your watch pages, and each format carries a different CPM (cost per thousand impressions). When you see an overlay running on a video, that’s called a YouTube InVideo ad, and advertisers pay a nice hefty CPM for that unit. When there’s not an InVideo ad sold, then you’ll only see the square ad to the right of the video. We call that a 300x250 (the dimensions of the ad), and the CPM is much lower.
Your net revenue depends on several things, including which ad format appears, what the advertiser paid, and the amount of times your video was viewed.
In addition, a viewer’s geographic location will impact your net revenue. We're steadily expanding the InVideo ad format internationally; this format generates the most revenue for you. In the beginning, it’ll be a little tricky to gauge what your monthly payment will be. Keep in mind that an advertiser has to be interested in advertising on your content, and it may take time to build interest."