iis7 rewrite & web.config for 404 error

I have this in my web.config:
<customErrors mode="On" defaultRedirect="/contact.aspx">
  <error statusCode="404" redirect="/contact.aspx" />
</customErrors>

But in my IIS7 it still says this in the module error pages:
404        %SystemDrive%\inetpub\custerr\<LANGUAGE-TAG>\404.htm

And the latter is the page that is shown when a 404 occurs.

Why is IIS ignoring the setting in my web.config?
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