Почему ::after назначается дочерним элментам

html,
body,
div,
span,
applet,
object,
iframe,
h1,
h2,
h3,
h4,
h5,
h6,
p,
blockquote,
pre,
a,
abbr,
acronym,
address,
big,
cite,
code,
del,
dfn,
em,
img,
ins,
kbd,
q,
s,
samp,
small,
strike,
strong,
sub,
sup,
tt,
var,
b,
u,
i,
center,
dl,
dt,
dd,
ol,
ul,
li,
fieldset,
form,
label,
legend,
table,
caption,
tbody,
tfoot,
thead,
tr,
th,
td,
article,
aside,
canvas,
details,
embed,
figure,
figcaption,
footer,
header,
hgroup,
menu,
nav,
output,
ruby,
section,
summary,
time,
mark,
audio,
video {
  margin: 0;
  padding: 0;
  border: 0;
  font-size: 100%;
  font: inherit;
  vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
menu,
nav,
section {
  display: block;
}
body {
  line-height: 1;
}
ol,
ul {
  list-style: none;
}
blockquote,
q {
  quotes: none;
}
blockquote:before,
blockquote:after,
q:before,
q:after {
  content: '';
  content: none;
}
table {
  border-collapse: collapse;
  border-spacing: 0;
}
.header {
  width: 100%;
}
.header-top {
  width: 100%;
}
.header-top_logo {
  display: inline-block;
}
.header-top_auth {
  display: inline-block;
}
.header-top_mobi {
  height: 50px;
  position: relative;
}
.header-top_mobi ::after {
  bottom: -5px;
  box-shadow: inset 0px 4px 8px -3px rgba(17, 17, 17, 0.06);
  content: "";
  height: 5px;
  left: 0px;
  opacity: 0;
  pointer-events: none;
  position: absolute;
  right: 0px;
  transition: opacity 0.4s;
  width: 100%;
  will-change: opacity;
}
.header-top_logo-mobi {
  width: 125px;
  height: 50px;
  position: absolute;
  left: 50%;
  margin-left: -62.5px;
}
.logo-mobi {
  width: 125px;
  height: 50px;
}
.header-top_auth-mobi {
  background-image: url("images/key.png");
  background-position: 0% 0%;
  background-repeat: no-repeat;
  background-size: 50px 50px;
  width: 50px;
  height: 50px;
  position: absolute;
  top: 0;
  right: 0;
}