Not even tabs usually live in the same process, so you must be misinterpreting something. I think, the Windows Task Manager groups processes in the most stupid

Not even tabs usually live in the same process, so you must be misinterpreting something. I think, the Windows Task Manager groups processes in the most stupid of ways, where when a (parent-)process has spawned many child-processes, then only the parent-process and its resource usage is shown. You have to go into the "Processes"-tab to see all Chrome-processes and manually add up their resource usage.