Bootstrap Drupal 8 core Telephone field css

I created a new form with a telephone field. In “Manage display” all labels are formatted as “above”. When I view my form the label of the telephone field is on the left. All other labels are above. The telephone input field is also not in 100% width but auto width.

I did not add any custom code.

It seems that to the telephone input field the 

@media (min-width: 768px)
.form-inline .form-control

is added. All other fields just use

.form-control

Why is there a @media selector and .form-inline applied to just this field?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed