PHP的排序对象返回了意外的顺序

按照单圈时间升序对车手数组进行排序:

function sortDrivers($a, $b)
{
  if (floatval($a->fastestLap) === floatval($b->fastestLap)) {
    return 0;
  }
  return floatval($a->fastestLap) < floatval($b->fastestLap) ? -1 : 1;
}
Log::info("Before:");
foreach($drivers as &$driver){
  Log::info($driver->driverCode . " - " . floatval($driver->fastestLap));
}
usort($drivers, "sortDrivers");
$position = 0;
foreach($drivers as &$driver){
  $driver->position = ++$position;
  $driver->save();
}
Log::info("After:");
foreach($drivers as &$driver){
  Log::info($driver->driverCode . " - " . floatval($driver->fastestLap) . " - " . $driver->position);
}

实际输出如下所示:

Before:  
Mark - 112.2  
Roger - 108.77  
Pete - 110.038  
Gary - 109.49  
Sid - 111.742  
Rod - 87.082  
Bob - 110.404  
Eric - 112.482  
Will - 113.478  
Nigel - 0  
Harry - 110.01  
Paul - 112.887  
Victor - 109.182  
Matt - 109.987  
Clive - 109.975  
Sol - 109.23  
Olly - 0  
Ben - 0  
Craig - 114.241  
Andy - 96.399  
Adam - 93.177  
Kel - 110.766  
After:  
Rod - 87.082 - 1  
Adam - 93.177 - 2  
Paul - 112.887 - 3  
Roger - 108.77 - 4  
Bob - 110.404 - 5  
Mark - 112.2 - 6  
Will - 113.478 - 7  
Gary - 109.49 - 8  
Pete - 110.038 - 9  
Harry - 110.01 - 10  
Matt - 109.987 - 11  
Craig - 114.241 - 12  
Eric - 112.482 - 13  
Sid - 111.742 - 14  
Kel - 110.766 - 15  
Clive - 109.975 - 16  
Sol - 109.23 - 17  
Victor - 109.182 - 18  
Andy - 96.399 - 19  
Olly - 0 - 20  
Nigel - 0 - 21  
Ben - 0 - 22  

虽然有明显的增加单圈时间的趋势，但一些车手的位置是错误的。

为什么?