Visual Studio 2013 Test Explorer does not always show all test cases

I'm using Visual Studio 2013 Pro with MSTest for unit tests. I have noticed something very odd in the Test Explorer.

If I run a test that has a datasource containing several test cases and they all pass, the Test Explorer will sometimes show only one test case in the results pane on the RH side, and sometimes it will show all the test cases individually as having passed. Repeatedly running the test will alternate between the two outputs.

Is this a bug in Test Explorer, or is something very clever going on that I haven't tumbled to?

TIA