Our Daily New User-count from Fabric is consistently bigger than the App Units per day that we get from iTunes Connect.
Fabric defines Daily New Users as
The number of new app installations across all devices seen on a given day.
And iTunes connect defines App Units as
The number of first-time app downloads made on the App Store using iOS 8 or tvOS 9, or later. App updates, downloads from the same Apple ID onto other devices, and redownloads to the same device are not counted. Family Sharing downloads are included for free apps, but not for paid apps.
What can cause this discrepancy? I can see many reasons for how Fabric could report a lower number (such as users downloading the app but never opening it, or waiting too long to open it), but not the other way around. Our average send-usage-to-developers opt-in rate is 23%, but that does not affect the App Units number if I am not mistaken.