Scalable horizontal navigation

I’m building a high cardinality horizontal navigation, similar to the one below:


My expectations, when I shrink the window, is that the items that I’m not able to expose go to the drop-down list (as the width decreases, support, deals, etc. begin to appear inside the 'more').

Surprisingly (for me), Microsoft adds another mechanism to their navigation (an arrow for the user to slide the items, and the more option is intact at the end):



I’m quite sure that they tested this. However, is it a matter of coherence of the component through all the cases (what’s in and what’s out of the drop-down is always the same) or there’s another reason?