For example, we have test tree:
1
  1.1
     1.1.1
     1.1.2
  1.2
     1.2.1
     1.2.2
2
  ...
N
Subgroup 1.1 is expanded.
All element are selected in displayed tree.
Then we unselect single element (for example 1.1.1).
Start tests. Tests from 1.1.1 will be started.
It occurs due to selection of parent group and etc till root group.
So, if root group is selected, the all tests from all subgroups will be started regardless of their selections.

From my point, if subgroup or test is select/deselect, the selection of parent groups should be updated.