断点使用下一个断点大小，为什么？

Question

我有一个这样的brekpoints.scss设置：

// Breakpoint viewport sizes and media queries.
//
// Breakpoints are defined as a map of (name: minimum width), order from small to large:
//
//    (xs: 0, sm: 576px, md: 768px)
//
// The map defined in the `$breakpoints` global variable is used as the `$breakpoints` argument by default.
// Name of the next breakpoint, or null for the last breakpoint.
//
//    >> breakpoint-next(sm)
//    md
//    >> breakpoint-next(sm, (xs: 0, sm: 576px, md: 768px))
//    md
//    >> breakpoint-next(sm, $breakpoint-names: (xs sm md))
//    md
@function breakpoint-next(
  $name,
  $breakpoints: $breakpoints,
  $breakpoint-names: map-keys($breakpoints)
) {
  $n: index($breakpoint-names, $name);
  @return if(
    $n < length($breakpoint-names),
    nth($breakpoint-names, $n + 1),
    null
  );
}
    
// Minimum breakpoint width. Null for the smallest (first) breakpoint.
//
//    >> breakpoint-min(sm, (xs: 0, sm: 576px, md: 768px))
//    576px
@function breakpoint-min($name, $breakpoints: $breakpoints) {
  $min: map-get($breakpoints, $name);
  @return if($min !=0, $min, null);
}
 
// Maximum breakpoint width. Null for the largest (last) breakpoint.
// The maximum value is calculated as the minimum of the next one less 0.1.
//
//    >> breakpoint-max(sm, (xs: 0, sm: 576px, md: 768px))
//    767px
@function breakpoint-max($name, $breakpoints: $breakpoints) {
  $next: breakpoint-next($name, $breakpoints);
  @return if($next, breakpoint-min($next, $breakpoints) - 1px, null);
}
    
// Returns a blank string if smallest breakpoint, otherwise returns the name with a dash infront.
// Useful for making responsive utilities.
//
//    >> breakpoint-infix(xs, (xs: 0, sm: 576px, md: 768px))
//    ""  (Returns a blank string)
//    >> breakpoint-infix(sm, (xs: 0, sm: 576px, md: 768px))
//    "-sm"
@function breakpoint-infix($name, $breakpoints: $breakpoints) {
  @return if(breakpoint-min($name, $breakpoints) ==null, '', '--#{$name}');
}
    
// Media of at least the minimum breakpoint width. No query for the smallest breakpoint.
// Makes the @content apply to the given breakpoint and wider.
@mixin media-breakpoint-up($name, $breakpoints: $breakpoints) {
  $min: breakpoint-min($name, $breakpoints);
  @if $min {
    @media (min-width: $min) {
      @content;
    }
  } @else {
    @content;
  }
}
    
// Media of at most the maximum breakpoint width. No query for the largest breakpoint.
// Makes the @content apply to the given breakpoint and narrower.
@mixin media-breakpoint-down($name, $breakpoints: $breakpoints) {
  $max: breakpoint-max($name, $breakpoints);
  @if $max {
    @media (max-width: $max) {
      @content;
    }
  } @else {
    @content;
  }
}
    
// Media that spans multiple breakpoint widths.
// Makes the @content apply between the min and max breakpoints
@mixin media-breakpoint-between($lower, $upper, $breakpoints: $breakpoints) {
  @include media-breakpoint-up($lower, $breakpoints) {
    @include media-breakpoint-down($upper, $breakpoints) {
      @content;
    }
  }
}
    
// Media between the breakpoint's minimum and maximum widths.
// No minimum for the smallest breakpoint, and no maximum for the largest one.
// Makes the @content apply only to the given breakpoint, not viewports any wider or narrower.
@mixin media-breakpoint-only($name, $breakpoints: $breakpoints) {
  @include media-breakpoint-between($name, $name, $breakpoints) {
    @content;
  }
}

然后在另一个变量文件中，我有

$breakpoints: (
  'xs': 0,
  'sm': 480px,
  'md': 768px,
  'lg': 992px,
  'xl': 1200px,
  'desktop': 1440px,
);
    
$snap: lg;

例如，当我使用 media-breakpoint-down(sm) 时，断点发生在767px 。 看起来它确实采用了下一个断点-1px的值。 但我试图理解为什么会这样做，而不仅仅是使用指定断点的实际值。