The MDS file is being changed by other users, but it's not specifically project variables.
Here is the scenario that seems to trigger checking out the project file.
User 1 has the current latest copy
User 2 has the current latest copy
User 1 checks out the project and modifies something (ie, adds a script, etc...)
User 1 checks in the changes
User 1 Selects Get Latest Changes. Since nothing is changed there is no dialog asking the user to check out the project.
User 2 Selects Get Latest Changes. TC indicates that the MDS file was modified outside of test complete and gives you the choice to reload it.
User 2 Selects Yes to the reload prompt. TC indicates that this action requires it to be checked out.
I am not sure why TC requires it to be checked out to reload it when getting all the latest changes.
