The Unit Tests pad allows multiple tests to be selected by using Command-Click. If you then Command-Ctrl-Click -> Run Test, only a single test is executed.
I've seen this behaviour on Xamarin Studio 5.10.3 and 6.0 (build 5104). With 6.0, the first (alphabetically) unit test is run. With 5.10.3, I think it might have been the unit test that you Command-Ctrl-Clicked on that was run but I'm not sure about that.
Ideally, we should be able to select and execute multiple unit tests within a solution/project/namespace/class or from different projects/namespaces/classes. If this is not possible then perhaps multiple selection should be disabled within the pad.
*** This bug has been marked as a duplicate of bug 27375 ***