1. You get an advertisement which google adsense had never advertises.
2. If Advertisement sender is gmail is different from what’s registered with google adsense.
example: you get mail from icicilombard but via thirdparty mail ID it’s spam, if you get it directly from icicilombard it’s not spam.
3. Google auto filter preferences, like sbicapsec will be always in spam! But ICICI direct never goes to spam, you can feel it’s same as first point, but I’m not sure.
4. Untill that mail is not marked as spam by some one it will be always in your inbox!
5. last but not least you might have set it as spam!