Navigation menu portlets are configured by default to show "unlimited sublevels" however only one sublevel is ever possible and when configuring the portlet to show 1 sublevel ... no sublevels display at all meaning the index is offset by one. Issue has been occurring since 7.0.
Almost a year has passed since this was marked as won't fix and it has existed as a bug for 3 years already
1) Create 5 non child pages
2) Create a navigation menu
3) Add these 5 pages to the navigation menu
4) Drag and drop to create a parent that has a child page
5) Add a child page to that child page
6) Add a child page to that child page
7) Parent > Child > Child > Child
8) Configure a navigation menu portlet using "Bar Minimally Styled"
9) Configure to show 1 sublevel ... no sublevels display at all
9) Configure to show 2 sublevels ... only 1 sublevel displays
10) Configure to show 3,4,5 or 6 ... only 1 sublevel ever displays
All child pages are navigable
Only one child page can be navigated to, the child pages below that child page are not visible no matter how many sublevels above 2 the user chooses to display.