After associating an AdSense account with your YouTube account, YouTube may deposit a bonus in your AdSense account to thank you for your video monetization contribution before you associated an AdSense account.
The bonus may be based on a number of factors including, among other things, your video performance, your account standing, any site-level invalid traffic analysis, and your previous relationship with AdSense, if any.
Technically, we can't accrue earnings for you until you associate an AdSense account. After associating your accounts, your future earnings from the ads served with your videos can be viewed in AdSense and in YouTube Analytics.