why one div with col-xs-8 and another div with col-xs-4 are not aligning in the same line code example

Example 1: row class in bootstrap

/*
In Bootstrap, the "row" class is used mainly to hold columns in it.
Bootstrap divides each row into a grid of 12 virtual columns. 
In the following example, the col-md-6 div will have the width of 6/12 of the "row"s div, meaning 50%. 
The col-md-4 will hold 33.3%, and the col-md-2 will hold the remaining 16.66%.
*/

<div class="row">
    <div class="col-md-6"></div>
    <div class="col-md-4"></div>
    <div class="col-md-2"></div>
</div>

Example 2: a column inside another column bootstrap html

<div class="row">
  <div class="col-sm-9">
    Level 1: .col-sm-9
    <div class="row">
      <div class="col-8 col-sm-6">
        Level 2: .col-8 .col-sm-6
      </div>
      <div class="col-4 col-sm-6">
        Level 2: .col-4 .col-sm-6
      </div>
    </div>
  </div>
</div>