php – 为什么我从两个假设类似的算术运算中得到不同的结果？

function get_value() { return 26.82; }

function a($quantity) {
    $value_excluding_vat = get_value();
    $value_including_vat = round($value_excluding_vat * (1 + (20 / 100)),2);
    $total_nett = 0;
    $total_gross = 0;
    for($i=0; $i<$quantity; $i++) {
        $total_nett += $value_excluding_vat;
        $total_gross += $value_including_vat;
    }
    return array(
        $total_nett,$total_gross
    );
}

function b($quantity) {
    $value_excluding_vat = get_value();
    $value_including_vat = round($value_excluding_vat * (1 + (20 / 100)),2);
    return array(
        $quantity * $value_excluding_vat,$quantity * $value_including_vat
    );
}

$totals = a(1000);
print_r($totals);
echo $totals[1] - $totals[0];
echo "nn";
$totals = b(1000);
print_r($totals);
echo $totals[1] - $totals[0];

这是我的输出：

Array
(
    [0] => 26820
    [1] => 32180
)
5360.0000000005

Array
(
    [0] => 26820
    [1] => 32180
)
5360