My guess is that you're weaker than your training partner at shoulder stuff.
If you bench properly - elbows tucked, your shoulders aren't involved so much. So, you can actually have pretty weak shoulders and still bench decently. You will get found out on incline, though. Decline has a smaller range and feels a lot like a board press to me (lower range). I would say that I am stronger on decline but I would never do decline instead of flat, first -
To me, it's a bit of a joke exercise.