Last updated: Dec 09, 2018 09:23
Pay attention to the History column of the reports:
The built-in side-by-side report is generated only for the latest execution of each test. The tests here are not latest.
There are three more executions for each test that followed.
By design, the "test set" is generated only for the latest reports and only for those that are no older than 7 days.
You can still manually select and compare the earlier reports by selecting the checkbox on the left hand side of the report name.