It would be a nightmare to have every file accurately list the contributors to that specific module (imagine maintaining that correctly over the years, as contributors come and go and others make edits or even major rewrites to your code, as bits of code move from file to file, etc.).

To simplify this, we chose to apply a uniform "(c) <primary contact> et al" style notice everywhere.  That would be me -- the primary contact for OIIO overall.  

Don't worry, you retain copyright to whatever code you contributed -- you're not assigning me the copyright. We're just using the simplified notice to avoid constant battles over exactly how a file-by-file-differing notice would be maintained.