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I noticed that my website was looking well on desktop, but on my phone something was off. In short, my website was of the form

<div class="row">
    <div class="col-sm-12"> <!-- content A --> </div>
    <div class="col-sm-12"> <!-- content B --> </div>
    <div class="col-sm-12"> <!-- content C --> </div>
<div class="row>
    <!-- content D -->
What I noticed: The second row had no white space on the left side.

First approach: changing content D to <div class="col-sm">content D</div>.

Nothing changed.

Second approach fixed it: changing content D to <div class="col-sm-12">content D</div>.  
It's worth noting that col-sm-4 has the same effect on the margin.

Now I'd like to know what the difference is between using no column, "col-sm", and "col-sm-12".  
col-sm ("Projects" is too far to the left):

col-sm-12:

Obviously, somehow the width of the header for "Projects" is changed, but I don't understand how.  
The reason you are not seeing the 15px padding when you use col-sm is because col-sm is not defined for Bootstrap 3 thus basically col-sm is not really having any effect in the padding. The col-* classes were introduced in Bootstrap 4. Below is an excerpt from bootstrap 3:
.col-xs-1, .col-sm-1, .col-md-1, .col-lg-1, .col-xs-2, .col-sm-2, .col-md-2, .col-lg-2, .col-xs-3, 
.col-sm-3, .col-md-3, .col-lg-3, .col-xs-4, .col-sm-4, .col-md-4, .col-lg-4, .col-xs-5, .col-sm-5, 
.col-md-5, .col-lg-5, .col-xs-6, .col-sm-6, .col-md-6, .col-lg-6, .col-xs-7, .col-sm-7, .col-md-7, 
.col-lg-7, .col-xs-8, .col-sm-8, .col-md-8, .col-lg-8, .col-xs-9, .col-sm-9, .col-md-9, .col-lg-9, 
.col-xs-10, .col-sm-10, .col-md-10, .col-lg-10, .col-xs-11, .col-sm-11, .col-md-11, .col-lg-11, 
.col-xs-12, .col-sm-12, .col-md-12, .col-lg-12 {
  position: relative;
  min-height: 1px;
  padding-right: 15px;
  padding-left: 15px;
As you can see, col-sm is not defined. If we look at bootstrap 4 though
.col-1, .col-2, .col-3, .col-4, .col-5, .col-6, .col-7, .col-8, .col-9, .col-10, .col-11, .col-12, 
.col, .col-auto, .col-sm-1, .col-sm-2, .col-sm-3, .col-sm-4, .col-sm-5, .col-sm-6, .col-sm-7, 
.col-sm-8, .col-sm-9, .col-sm-10, .col-sm-11, .col-sm-12, .col-sm, .col-sm-auto, .col-md-1, .col-md-2,
.col-md-3, .col-md-4, .col-md-5, .col-md-6, .col-md-7, .col-md-8, .col-md-9, .col-md-10, .col-md-11, 
.col-md-12, .col-md, .col-md-auto, .col-lg-1, .col-lg-2, .col-lg-3, .col-lg-4, .col-lg-5, .col-lg-6, 
.col-lg-7, .col-lg-8, .col-lg-9, .col-lg-10, .col-lg-11, .col-lg-12, .col-lg, .col-lg-auto, 
.col-xl-1, .col-xl-2, .col-xl-3, .col-xl-4, .col-xl-5, .col-xl-6, .col-xl-7, .col-xl-8, .col-xl-9,
.col-xl-10, .col-xl-11, .col-xl-12, .col-xl,
.col-xl-auto {
  position: relative;
  width: 100%;
  min-height: 1px;
  padding-right: 15px;
  padding-left: 15px;
There you can see col-sm right after col-sm-12
        
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