I finally got some time to write some code to show this...I 
may be wrong in assuming the behaviour, but either way the 
totals are definately wrong.  Here's the snippet:

$value = 1234;

$total = $value + $value;
print number_format($value,2) . " + " . 
number_format($value,2) . " = " . number_format($total,2);

print "<BR><BR>";

$value = number_format($value,2);
$total = $value + $value;
print $value . " + " . $value . " = " . $total;