How are Sales different from In-App Purchases in iTunes connect? In my app, I have 1 In-App Purchase. In App Analytics, in Metrics:

My Sales and In-App Purchases are different. For so many days, I have one or more In-App Purchase but no Sale.

How is this possible?

Edit:
Sales should include the revenue from In-App Purchases. From Apple Help, Sales mean, "Sales is the estimated total amount billed to the customers.
Quoting from, About in-app purchases:


  In-app purchases are extra content or subscriptions that you can buy in apps on your iOS device or computer.


It's likely that existing customers, who have previously bought and installed the app have made the in-app purchases during the said period.

In-app purchases can be made at any later point (as available) after installing the app. Also, for paid apps, downloading results in a sale only upon first time installation by user.