You are correct: Pages at the same level in the hierarchy are sorted alphabetically, you can move pages up and down in the hierarchy but you cannot change their relative ordering.
The motivation for this design was to allow you to categorize your wiki without having to worry about the precise ordering of pages within a category.
Edit to clarify the design motivation:
Let's say I'm working on a feature called Wiki Navigation and I create some wiki pages for it. With the wiki hierarchy I can create a set of pages like:
- Wiki Navigation
- Wiki Nav Function Specification
- Wiki Nav Mockups
- Wiki Nav Technical Specification
- Backfilling ixWikiPageParent
Which is handy, because the pages are grouped together and I can get from one to the other relatively easily using the the little "related pages" widget and breadcrumbs. It does not let me change the ordering of a given level which I'd argue is a good thing because: for the most part, I don't care about the order, It saves me any time I might have spent neurotically alphabetizing pages when they accidentally get out of order and it makes the editor simpler and more reliable.
That's not much solace in those situations where you do want to control the sort order, but this is a situation where we traded control for ease of use and efficiency.