Is this seen in all XFA docs or just this one?
Does this happen in AcroForms also?

The XFA spec in Ch 14 'Accessibility and Field Navigation' specifies how default tabbing is supposed to work; left->right, then top->bottom. Though that doesn't handle RTL languages gracefully, and having spent some time on other projects dealing with tab ordering, what is the nearest item to the right, etc can be tricky. Additionally, there is a mechanism to specify the traversal order, both on the elements directly or via a script.

I am suspecting we are either just ignoring all of the traverse stuff and doing something stupid like using instantiation order, have screwed up the next item calculation in the default case, or the doc has screwy traverse instructions.

It's not a problem with the doc, since in Adobe Reader the order is predictable (left to right, top to bottom).

This doc is just an example, other docs have the same problem.