Tabellenveränderung bei mouseover

Hallo, hat jemand eine Idee, warum der rote Bereich schmaler wird, wenn die Maus über der Tabelle ist?

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    <!DOCTYPE html>
<html>
<head>
    <title></title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="content-type" content="text/html; charset=utf-8" />
    <meta http-equiv="Content-Language" content="de-DE" /> 
    <style>
        body {}
        
        table {
            border-collapse: collapse;
            width: 100%;
        }
        
        tr.hidden {
            display: none;
        }
        
        table.t1,
        tr,
        td,
        th {
            border: 1px solid #999;
        }
        
        td,
        th {
            font-size: 82%;
            /*75%; x-small*/
            padding: 2px;
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