okay, so i just mark the top face and the bottom one of my cube so i expect the Median Pivot Point average their normals and will be oriented towards the right face, the left face, rear face or front one as perpendicular on the average normals of the top and bottom faces .... but that's actually what i get about the pivot position , Weird !! How it's put like that , the average is not what i expected ?! 
Boula, I love how thoroughly you're going through this course! This honestly looks like a bug, I would expect that same thing that you expected. I'll report it to the developers.
Edit: I'm not sure what could be done though, since the Z and Y axes are pointing in the direct opposite directions. Their average is zero, so those two are technically correct. And you can't change the X axis without changing one of the other two. So it's not wrong per se, but it's not very useful.