How to sum views field value

A have Drupal 8 and a custom module, which adds a custom field to a view.
I want to sum two fields in each views row.

In my module I sum two field values:

  public function render(ResultRow $values) {

  $value = $this->view->field('field_one')->original_value;
  $value_two = $this->view->field('field_two')->original_value;
  $sum = $value + $value_two;
  return $sum;

}

but I get a number 2 as a result for all rows instead of real sum. Looks like a count, not a sum.

Why so that?