On second thought, i don't think this is due to char value sorting at all. below is how the doc browser is sorting. The two filenames with "_c_" should be next to each other, as they DO show in the plugin file browser, likes values next to like values, regardless of any character set char values for a 'c' versus an underscore.
show_area_c_.php show_area_cc.php show_area_cc_rs.php show_area_c_rs.php