The reason for this is the review process of Adsense. Adsense has its own way to review the sites. The review steps of Adsense is divided into three steps. Each steps is done cautiously. They are described below;
1. First step;The first review process is done by the Adsense bots. These bots are very intelligent machine powered by the ideas of the finest men around. The whole part if your blog is scanned thoroughly in this step. If the site is good and matches with the policy it approved the site and this you receive a mail of confirmation.
If not then you will receive a mail of declination. As the bots are just machines their might be rare cases of acceptance of spam or fake blogs also in this step.
2. Second Step;After the first review step Adsense requests you to implement the ad code. Unless you implement your ad code the review process would not proceed. After the implementation of ads the second step starts. Here you won't see the ad you kept as it was just to verify you as it's owner.
In this step real people check upon your site. If the site is good your site is accepted and if not you would be tossed aside.
Upon accepting your blog, the ad you implement would show up. As soon as it shows up it is confirmed that you have been accepted in the review process.
3. Third Step;this step is the never ending review process done by the bots. Even after you start earning revenues your blog is continuously scanned along with all other sites by the bots. It is the safety measure to make sure your site is free of plagiarism now and in future. Even after you start earning money if Adsense bots detects any sort of plagiarism in your blog then they would disqualify you.
So these were the review process of Adsense explained by me. With time running on Adsense would certainly tighten its policy and so would the review process be more strict. So always be honest with Adsense.