Ok, I was able to produce a similar result here and understand what is going on. The problem is that the page is a child page of Web Shops and even though it is not included in the menu it is making room for the menu because it knows that the Web Shops page has children but it doesn't know they are not included in the menu.
I was able to modify the code here to detect that particular scenario and not make room for the menu but it isn't going to handle every possible strange combination. Like if Web Shops has other child pages that are included in the menu then the menu will be shown even with my fix. Also if you create child pages below a page and then you set the parent page as not included in the menu then the children pages will also not appear.
So there will be a solution for this in the next release that should solve this specific issue. In the menatime I would either include the menu or move the page up a level so it is in the horizontal menu and has no children. In other words you can make it parent Root and mark it as not included in the menu and it will use the full width.
Hope it helps,