Symptoms

Scenario:

Let's assume that there are 3 Processing Stations - "a", "b" и "c," and also two workflows "1" and "2". Stations "a" and "b" are united into a single group and is assigned workflow "1". It is expected, that the documents for workflow 2 will be processed only via Station "c". In fact, for workflow "1" and "2" all stations are available.

Cause

Group behavior:
By default, all Processing Stations can be reached by any workflow. 
With the help of a group, we add a limitation for a workflow, making it available for a specific group of Stations. 

Resolution

You would need to create another group of stations and assign the workflows that you have left to that second group. This way, you will have your original group of stations isolated from the use by other workflows.