Not necessary. For example when you use a music app like Play Music or Power Amp your media server spikes. The app might have a minimal use in battery stats but its using a google services and making Android System spike. As to why it isn't consistent I don't know. My Android system is always under 9%. Best I can say is trouble shoot with all your apps. What do they do what permissions and figure out wich of them might be relative to the services in Android System.