How does Office for the web determine the datacenter to route a given user to?
When using Office for the web to view and edit documents stored in external (non-Microsoft) storage, users are typically routed to the Office for the web datacenter closest to their current geographic location. However, this routing is not guaranteed, and users may be routed to any global datacenter for a variety of reasons.
Thus, Office for the web does not guarantee that data will stay within a particular geographical region when used to view and edit documents stored in external (non-Microsoft) storage.