fpdf php：字符宽度不是pts？

There are 72 computer pts to an inch. The widths of fonts in the $cw array that carries character widths is typically between 270 and 600 per character.

A4 = 11.7 inches width in landscape 11.7 * 72 = 842.4 pts

So what does the 'character length' in $this->CurrentFont['cw'] actually represent?

Examples: chr(0)=>278,chr(1)=>278,chr(2)=>278

I've got it working using:

$curCharLength = $charWidthsInPts[$curChar]*$this->FontSizePt/1000;

$str_len += $curCharLength;

I compare this to the width in pts. However I don't see how they arrived at these numbers.