Ever wondered why the custom properties (such as author, title, subject, etc) associated with a file are automatically picked up during the upload process?

 This is due to the "Document Property Promotion and Demotion" feature available in SharePoint.

I quote from this MSDN article which talks about that,-

"Managing the metadata associated with your document is one of the most powerful advantages of storing your enterprise content in Windows SharePoint Services 3.0. However, synchronizing information between the document library level and the document is a challenge. Windows SharePoint Services provides the document parser infrastructure, which enables you to create and install custom document parsers that can parse your custom file types and update a document for changes made at the document library level, or update the document library with changes made at the document level. Using a document parser for your custom file types helps ensure that your document metadata is always current and synchronized between the document library and the document itself."