GD bounding box incorrect for text (php) - php

Question

I'm having issues with text lining up where it should. Every tutorial I can find tells me to use imagettfbox to find the dimensions of the text. I do this, but the text doesn't fit within those dimensions.
See the example below:
<?php
// Create image and colours
$im = imagecreatetruecolor(200, 100);
$white = imagecolorallocate($im, 255, 255, 255);
$black = imagecolorallocate($im, 0, 0, 0);
$red = imagecolorallocate($im, 255, 0, 0);
//Set the background to be white
imagefilledrectangle($im, 0, 0, 200, 100, $white);
//font file
$font = "ARIAL.TTF";
//set sample text and fontsize
$text="102";
$size = 25;
//calculate height and width of text1
$bbox = imagettfbbox($size, 0, $font, $text);
$width = $bbox[2] - $bbox[0];
$height = $bbox[5] - $bbox[3];
//draw rectangle that should house the text
imagefilledrectangle($im, 50, 40, 50+$width, 40+$height, $red);
//draw text at same startpoint as rectangle
imagettftext($im, $size, 0, 50, 40, $black, $font, $text);
// Output to browser
header('Content-type: image/png');
imagepng($im);
imagedestroy($im);
?>
This should output a red rectangle, containing the text. It doesn't - the text is slightly to the right of where it should be. This only happens with some strings - ones that contain a "1" tend not to work, while some others work perfectly. Here is the output for 2 strings:
Does anyone know what is happening, and what I need to do to make the text fit correctly within the bounding box? I've tested this with multiple fonts, including monospace fonts - the same thing always happens.