Can't understand negative margin's way of work

Please look at following html...

.wrapper {
  position: relative;
}

.mydiv1 {
  margin-left: -10px;
  margin-top: -10px;
}

<div class="wrapper">
  <div class="mydiv1">div1</div>
</div>

.mydiv1 was pulled left as 10px and through out it's parent's left border. But nothing happens on its top. Why did it work differently?