css optimizer uses both core and override css

I use my theme's .info file to override core css files (e.g. system.css) as described here: http://drupal.org/node/171209

Override usually works: Only sites/all/themes/theme/css/override/system.css is used, as long as I don't enable css optimization at /admin/settings/performance. If I do, core and override (modules/system/system.css and sites/all/themes/theme/css/override/system.css) files are included in aggregate css. I verified this behavior by adding dsm($file); to /includes/common.inc in line #1767.

I'm running Drupal 6.2 on Ubuntu 7.10 JeOS, using Lighttpd 1.4.18 (w/ mod_magnet)/PHP 5.2.3 (FastCGI)/MySQL 5.0.51. The only activated Drupal module is devel, /var/www is exported from a Mac via HGFS (r/w).