In Lloyd McKenzie's words:

In Questionnaire Response, the nesting of questions following the following rules:

• for questions, you always nest beneath answer
• for groups, you nest directly beneath the item

In both cases, the hierarchy is retained. The reason for the difference is that when there are multiple answers for the parent question, you need to do the nesting for each answer.

There is some documentation in the SDC IG, but this needs to be surfaced in the documentatino for the Questionnaire Response.