Z-index works with position static, why?

Question

I always thought that the z-index doesn't work with position static, but this is not entirely true, you can open my example on the Codepen and try to uncomment the z-index, then watch the behavior of the elements on hover. If you want to see a bug you have to hover over the element in the first row. Сan someone explain this behavior?

.container {
  display: grid;
  justify-content: center;
  grid-template-columns: repeat(2, 210px);
}

.card {
  width: 200px;
  padding: 5px;
  margin: 5px;
  transition: 1s;
}

.card:hover {
  z-index: -1;
  background: cyan;
  transform: scale(1.2);
}

.card__img {
  width: 200px;
  border-radius: 50%;
}

.card__p {
  padding: 5px;
}

<div class="container">
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
  <div class="card">
    <img 
      src="https://static3.depositphotos.com/1000992/133/i/950/depositphotos_1337508-stock-photo-a-free-flying-white-dove.jpg"
      class="card__img"
      alt=""
    >
    <p class="card__p">
      Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    </p>
  </div>
</div>