As a default, the Page uses Asset Rank for the order for Assets in a given location on the page. However, once anything has been placed using the drag interface, then it is fixed, and Asset Rank is ignored.
I was able to duplicate what you're seeing. If I add a SQL Report, and move it on the front page, it will move AND change rank once. After that, only the page position will change.
Similarly with Asset Rank. If you change rank, it will change the order on the page until ANY asset on the page is dragged. Then it doesn't matter anymore.
Closing as not a bug.