It may be that revenue continues to arrive for a long time in small amounts even after deleting your release. This income is incurred when a customer purchased one or more of your tracks and stored them in a Cloud Locker service (iTunes Match for instance).

 A cloud locker saves a listener's local music files into a cloud allowing them to be streamed from other devices, such as mobile phones or tablets, or downloaded back to the computer. Most vendors will match the users music library to a music store (iTunes/Amazon/etc.) so that each file doesn’t need to be separately uploaded. Each download and stream from the cloud will lead to compensation, however it’ll be significantly lower than that which comes from normal streams. 

If you delete your releases at some point, customers who’ve purchased your tracks will still have them saved in their cloud. You can compare this to when you purchase a book- even if it’s no longer published, those who’ve previously purchased it will still have a copy. Likewise, each time a customer plays your music from their cloud, the store will register this and credit you with small amounts.Cloud locker plays are also registered even if you don’t provide streaming services- and you’ll notice the streaming revenue from this in your trends.