Cached tasks are the ones reusing shared state objects. 

To reproduce:

1. In managed mode, run a clean build of core-image-minimal 
2. Once the build is finished, delete the /tmp directory 
3. Build again core-image-minimal. This build uses shared state objects for a whole bunch of tasks.
4. When the build finishes, visit the tasks table and click the filter button next to the outcome column heading. 

Outcome: the number of 'Cached tasks' is 0

Expected outcome: the cooker log for the build shows 720 successful setscene tasks, so I would expect to see 720 Cached tasks in Toaster. The cached tasks seem to be counted as 'Covered' tasks

By the way, this happens in both managed and interactive modes.