Unusual spikes in the number of downloads
We saw some artifacts on Nov 13 and 29 2017 from App Annie’s download number, and some spikes from their promotion number in other days. In fact, the units reported by iTunes Connect "Sales and Trends" (when the time zone is set to PST) is equal to the sum of App Annie's downloads and promotions under the "Downloads" tab (see https://www.mediawiki.org/wiki/Wikimedia_Apps/Team/iOS/Analysis#Browse_and_Install for more details). And on iTunes Connect, if we breakdown the number by device, we could see those unusual bumps all came from desktop.
Based on our email exchange with App Annie (see below) and this discussion on Apple Developer Forums, these downloads from desktop are most likely from Volume Purchase Program, either for educational institutions, or for large enterprises. The way these institutions distribute apps to staff internally is via some form of Mobile Device Management system. There are apps they have to distribute to staff/students whatever, and these are purchased in a way that shows up as a desktop purchase in the stats. This kinda makes sense as the person purchasing your app is most likely sat at a desktop computer putting the order through the system. Unlike a user purchasing and downloading your app, this is like a 'right to download' so many copies of your app, and not actual downloads.
Although Volume Purchase Program can't explain all the spikes, we consider downloading app on desktop is not a usual behavior, especially since version 12.7 Apple removes the desktop iOS app store. 
Therefore, we should exclude downloads on desktop from the downloads number we are reporting.
Email with App Annie:
> Question: Can you explain what "promotion" mean for a free app like us (Wikipedia app)? Thanks!
> Answer: ... Promotions are downloads that are made using a promotional code, and these codes can be obtained in several ways. The traditional use of Promotions was with codes that you could generate yourself, and then distribute to end users. Typically, this would be done to give someone a promotional copy of your app. Later Apple expanded the use of promotional codes to include their Volume Purchase Program. When someone makes a bulk purchase of an app, Apple provides them with promotional codes that they can then distribute to end users to download your app. This enables the end user to download the app without needing their own iTunes account to do so. It is very popular with educational apps, where a school might want all students to have access to an app. They purchase the app in bulk, and distribute the codes to the students so they can download the app onto their devices. For more information on Apple's Volume Purchase Program, please see their official site here: https://vpp.itunes.apple.com/
Purchases such as these show up as "Desktop Downloads" in the Sales and Trends section of your iTunes Connect.