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.
You mean I have to actually read those permissions ? I always just say yes. Well with 49 apps I have a lot to read, lol. However, wouldn't it have to be one of these showing under "running" in manage apps?