Several versions can be associated with a given model. Additionally, if you remove a Start or End node you will be forced to create a new model version. This is to prevent breaking any subflow dependencies, i.e if an end node that is used to link to another subflow within a master model is removed, then the master model will break.
Each time a model is saved, it is saved either to an existing model version, or a new version is created. Model versions do not appear in the Test Modeller "Models" workspace, but are available to be viewed once the model has been opened in the Model Editor.
To browse between versions of an open model, click the "Version" button in the home tab of the menu. To open and edit a given Version, click the eye icon next to it: