Why are bootstrap 4 grid-breakpoints not matching?

Question

Sass

$grid-breakpoints: (xs: 0, sm: 576px, md: 768px, lg: 992px, xl: 1200px)
$container-max-widths: (sm: 540px, md: 720px, lg: 960px, xl: 1140px)
// in sass write
+media-breakpoint-down(lg)
.login
margin-right: 0
//expect to see
media (max-width: 992px)
.login {
margin-right: 0;
}
//but see
media (max-width: 1199.98px)
.login {
margin-right: 0;
}