Apparently, the System Preferences window on my computer running OS X El Capitan was considerably wider than 668 (or even 595) pixels. Some research later it turned out that the width of the System Preferences window depends on the system language: The window will be 668 pixels wide when English is the primary language, but will have a different width for other languages. For example, with German set as primary language, as it is on my computer, the window is 762 pixels wide.
While Apple’s preference panes that come with OS X dynamically adjust their width to the actual width of the System Preferences window, this is not true for any custom preference pane, that will instead be displayed centered with more or less wide blank margins.
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