Because "this" isn't the deployment, it's the Service object -- if we had 2 deployments, then self.hostgroups would return a hostgroup for each deployment (assuming the service was included in both). For example, if this is the nova-compute service, and we had 2 nova deployments, then self.hostgroups would return 2 hostgroups: the compute hostgroup for deployment 1, and the compute hostgroup for deployment 2. This moves in the direction of making this code multi-deployment-safe, although it might conflict with what you're doing in that other PR -- so we'll need to reconcile the two sets of changes somehow.