Why do I see more installs than “add-app” clicks?

There are cases when there are more installs than “add app” clicks. How is this possible?

•  Missing Google Analytics data for part of the period viewed. This is the most common reason. We get Shopify data, including Installs, for any period you choose to view. For some periods, Google Analytics data that contains “add app” clicks may be missing. 
• Bypassing the app store page from your landing page. Some app developers use smart links or other means to bypass the app store page and go straight to the install screen. In such cases, there is no “add-app” click but there are installs.
• Multiple installs across different stores. Merchants with multiple stores may install your app across several stores in a single session. Some add-app clicks lead to installs in several different stores.