Testflight metrics - installs and 7 days

From TestFlight tab in AppStore Connect, I'm interested in 2 column - INSTALLS and 7 days.

I've read the related document https://help.apple.com/app-store-connect/#/dev78d0a8f87.

I'm curious in 2 questions
1) the count of INSTALLS seems to not match with the number of External Tester whose status is "Installed 1.0.0 (1)". Why is this happen?

2) How "7 days" is represented?
If we use 2 devices with the same apple id to install the app, is this counted as 1 unit or 2 unit in the "7-days" metrics

"In the Status column, view the status of a build. In the other columns, view the number of invited testers, number of installations on devices, and the total number of sessions in the last seven days across all testers. In the Crashes column, view the number of times the build crashed on tester devices. In the Feedback column, for iOS builds, view the number of feedback submissions you received from your testers."

https://help.apple.com/app-store-connect/#/dev78d0a8f87 > View build status and metrics
Testflight metrics - installs and 7 days
 
 
Q