Please view the screenshots attached to this bug report and notice that, from what the UI is presenting, it's easy to interpret that Package Manager doesn't see Cinemachine as an installed package.
- notice that it's not listed in the "In Project List"
- notice it IS listed in the All Packages tab and you get the option to install it

Repro steps:
- create a project witht he 3D template
- pick Cinemacine from the Asset Package list
( I also picked UnityChan, but that's not relevant..)
-Load up the project then got to the Package Manager window and notice the discrepancy

Tested with Unity 2018.1 0b8