Uploading new media to existing posts/pages backdates file location

Sorry if the subject is a little obtuse — it’s hard to explain in one line.

Using 2.7.1, if I upload a new media file to a page or post that has already been published, the new file is added in the
/wp-content/uploads/year/month/
folder that corresponds to the original page/post publication date, rather than the upload date.

For example: Page was created on 9/1/2008 and the file was uploaded on 3/20/2009, previous versions of WordPress would have placed the file in:
/wp-content/uploads/2009/03/
however, my install of 2.7.1 is placing it in:
/wp-content/uploads/2008/09/.

Is this a known problem, a problem with my upgrade/install, or something else?