Here's the logic behind how these pages rank differently:
Creating a new story from Plan > User Stories page assumes it was created as a one-off and could easily be lost. Agile Central ranks it at the top of the Backlog so the Product Owner remembers to properly rank the one-off story. Basically, when a new story is created from outside the Backlog page, it will automatically be given the top rank. The reason this happens is so that the story isn't lost. Agile Central assumes stories created from other areas may not be fully fleshed out, and need to be ranked against the rest of the backlog, so it puts it in the most visible place -- at the top.
When creating a story on the Backlog page, Agile Central assumes that the user has an eye on the non-emergency story. Because the person should know that the story was created (and won't lose it), they'll know to immediately rank it after creating it.