Why is "out of grouping" announced when leaving a group in browse mode but not focus mode?

[awildash]

I was going to raise a bug but I noticed JAWS does this too so I think this might just be my lack of understanding of how grouping is in interpreted. So if you go here, for example:

https://www.w3.org/WAI/WCAG20/Techniques/working-examples/ARIA17/grouping-roles-example.html

And you:

• Use browse mode (arrow keys) to go down the page markup
• When you hit the 12 digit controls you'll here "grouping 12 digit account number"
• When you keep going and leave that group you'll hear "out of grouping"

But if you

• Tab into the 12 digit group you're informed its a group (thats fine)
• When you tab out of the last control from the group to the next group you dont hear "out of grouping"

In this particular case its obvious I suppose that you've left the grouping as it will announce the next grouping. But in a scenario where the next thing is a button, for example, it seems less clear to me. Or is it just a given / expectation that the next thing you focus on should be clear enough that its not part of the grouping and therefore it doesnt need to announce you've left the group? But then why is it announced in browse mode?