当我试图找到文本长度和字符长度之和时，PHP imagettfbbox给出了不同的结果

I have a string with font and i need to find it's width, and then width of each char in this string, I'm using GD lib imagettfbbox for this, but it always gives me different result in the end.

$text="some random text";
$info=imagettfbbox(18,0,'/var/www/html/ARIAL.TTF',$text);
$text_width=$info[2]-$info[0];

$text_width_by_chars=0;
for($i=0;$i<strlen($text); $i++){
    $char=$text[$i];
    $info=imagettfbbox(18,0,'/var/www/html/ARIAL.TTF',$char);
    $char_width=$info[2]-$info[0];
    $text_width_by_chars+=$char_width;
}
echo $text_width.' '.$text_width_by_chars;

It happens to all fonts doesnt metter if $info[0] posetive or negative or 0, the differance is always around strlen($text) but not exactly, i've tried to find out if i can mutiply char width to some multiplier but it doesn't work, it's always around 0.91. Why does it happen? And how can i prevent this so in the end $text_width and $text_width_by_chars become equal?