If you activate edit mode in the grade book, the "Controls"-Field has a wrong colpsan (colspan='1' if no idnumber is shown, colspan='2' otherwise), see screenshots 2)(with cell boundaries highlighted).

/cvsroot/moodle/moodle/grade/report/grader/lib.php
lines 1300 following:
$colspan = '';
if ($showuseridnumber)

{ $colspan = 'colspan="2"'; }

if (!$fixedstudents)

{ $iconshtml .= '<th class="header c0 controls" scope="row" '.$colspan.'>'.$this->get_lang_string('controls','grades').'</th>'; }

should be:
$colspan = 'colspan="2"';
if ($showuseridnumber) { $colspan = 'colspan="3"'; }

if (!$fixedstudents) { $iconshtml .= '<th class="header c0 controls" scope="row" '.$colspan.'>'.$this->get_lang_string('controls','grades').'</th>'; }

I assume, a column was introduced and the code is old code.
Similar lines (similar error??) are at lines 1173 and 1263.
Screenshot 3) shows the right cell boundaries.