Workspaces change when opening a project from someone else.
When opening a project that comes from someone else, all of their workspaces load, as well as the generic factory workspaces along with mine. Most of these I do not want to appear in the workspace window, so I have to go through the process of deleting them all, as the majority of the workspaces I use are custom created.
Visual Feast commented
That's actually an option you can turn off if you don't want it. Window > Workspaces > uncheck "Import Workspace from Project"