On Tot,
- navigate to Docs
- alt+f
- arrow down

result:
there's lots of re-parenting occurring. In particular,

(for an unselected menu item)
application -> menu -> menu item (active desc)
becomes

(when selected)
application -> group -> div -> ... -> menu item -> menu item (active desc)

The active descendant's ancestry chain changes when it gets selected via down arrow. Once selected, it has no sibling. Once unselected, it gets placed back into the first hierarachy where it has all sibling (except for the selected menu item).

The final result is that ChromeVox says
"open ... 1 of 1".