If I understand correctly, a level of review is represented by a gateway on the process map, and one user reviews the document (via an Assign Task operation) in each branch in the gateway.
Each Assign Task operation needs to use a different variable to save output. If each Assign Task operation uses the same variable as output, each time a user completes their review, their data overwrites the previous user's data.
When the gateway completes, you need to retrieve the comments from each variable that captures Assign Task output, and save them in a single variable. The gateway does not do this automatically. You can use a Set Value operation to accomplish this.
Hope that helps
Scott