Difference with quoted strings in binary operations

[xzyfer]

foo {
  bar: 3 - "hello";
  bar: (3 / "hello");
}

Ruby Sass

foo {
  bar: 3-"hello";
  bar: 3/"hello"; }

LibSass

foo {
  bar: "3-hello";
  bar: "3/hello"; }

Spec sass/sass-spec#459