Ever since the left top menu UIXP was introduced, it featured an order which is documented as:
the order in which the item will be displayed. The lower the number is, the more on the left the item is.
However, at the skin level, all top menu items (.navbar-nav > li) are floated right. That fits well the ordering logic of the right top menu, which is reversed:
The lower the number is, the more on the right the item is.
However, for the left top menu UIXP, it ends up reversing the documented and originally intended order.
So 3 UIX for org.xwiki.platform.topmenu.left using the orders "1, 2, 3", even if retrieved and handled correctly by the UIXP will be displayed as "3, 2, 1" because of the skin's bad CSS which did not consider the 2 types of top menus (navbar-left and navbar-right) and treating all items as part of a single, right floated, top menu.
The less/cass code in issue is at: