https://confluence.atlassian.com/display/SERVICEDESK/JIRA+Service+Desk+permissions

Update the documentation to reflect that normal JIRA Project Permissions are not honored in the same way with Service Desk Projects as they are in typical JIRA Projects. This is reflected in these two Service Desk Feature Requests:

JSD-861: Jira Service Desk should not restrict Project actions
JSD-931: Allow collaborators to modify issues dependent on their permissions

The documentation does state:
Each type of user displayed on the People tab of your service desk matches a JIRA project role, and the project permissions for each type of user are assigned to the JIRA project roles in the permission scheme

However, it should be clear that just because a certain permission is granted in the Project's Permission Scheme, it doesn't mean that the permission is granted in the Service Desk Project according to the Service Desk Project Roles, and that the Service Desk Project Permission Scheme does not take precedence.