css - PX-REM mixin only working with font-sizing -

so using mixin generate px value fallbacks when using rem unit:

@function fix8_unit_to_px($val) {     @if unitless($val) {         @if $val == 0 {             @return $val         } @else {             @return $val * 10+px         };     } @else { @return $val}; }  @function fix8_unit_to_rem($val) {     @if unitless($val) {         @if $val == 0 {             @return $val         } @else {             @return $val+rem         };     } @else { @return $val}; }  @mixin rem($prop, $val...) {     $n: length($val);     $i: 1;      $px-list: ();     $rem-list: ();      @while $i <= $n {         $px-list: append($px-list, fix8_unit_to_px(nth($val, $i)));         $rem-list: append($rem-list, fix8_unit_to_rem(nth($val, $i)));         $i: $i + 1;     }      @if $px-list == $rem-list {     /* 8 */         #{$prop}: $px-list;  /* 9 */     } @else {        #{$prop}: $px-list;  /* 9 */        #{$prop}: $rem-list; /* 9 */     } } 

this works if use on font-sizing i.e.

@include rem(font-sizing, 1.4); 

returns

font-sizing: 14px; font-sizing: 1.4rem; 

however if use other font-sizing, returns rem value:

@include rem(margin-top, 1.4); 

returns

margin-top: 1.4rem; 

why happening?