Why should context node type affect generation of a property name?

Hello,

I have an example of JSON document with a context, and I don't fully
understand how its compacted JSON-LD is generated.

Here it is. Note that the "versions" property has both @id and @type.

{
    "@context": {
      "bridge": "https://confluence.nrk.no/display/RED/Ordbok#",
      "schema": "http://schema.org/",
      "versions": {
        "@id": "bridge:versions",
        "@type": "bridge:dictionary"
      },
      "items": {
        "@id": "bridge:versions",
        "@container": "@index"
      },
      "name": "schema:name",
      "version": "http://semver.org/spec/v2.0.0.html#semver",
      "description": "schema:description"
    },
    "@id": "http://localhost/api",
    "versions": {
      "@id": "http://localhost/api/versions",
      "@type": "bridge:dictionary",
      "items": {
        "v1": {
          "@id": "http://localhost/api/v1",
          "@type": "api",
          "name": "v1",
          "version": "1.0.0",
          "description": "Initial release"
        },
        "v2": {
          "@id": "http://localhost/api/v2",
          "@type": "api",
          "name": "v2",
          "version": "2.0.0",
          "description": "Includes pizza-ordering service"
        }
      }
    }
  }

If I extract the context from the document and paster into a JSON-LD
Playground Context field, then the Compacted JSON-LD version is generated
like this:

{
  "@context": {
    "bridge": "https://confluence.nrk.no/display/RED/Ordbok#",
    "schema": "http://schema.org/",
    "versions": {
      "@id": "bridge:versions",
      "@type": "bridge:dictionary"
    },
    "items": {
      "@id": "bridge:versions",
      "@container": "@index"
    },
    "name": "schema:name",
    "version": "http://semver.org/spec/v2.0.0.html#semver",
    "description": "schema:description"
  },
  "@id": "http://localhost/api",
  "bridge:versions": {
    "@id": "http://localhost/api/versions",
    "@type": "bridge:dictionary",
    "items": {
      "v1": {
        "@id": "http://localhost/api/v1",
        "@type": "http://json-ld.org/playground/api",
        "description": "Initial release",
        "name": "v1",
        "version": "1.0.0"
      },
      "v2": {
        "@id": "http://localhost/api/v2",
        "@type": "http://json-ld.org/playground/api",
        "description": "Includes pizza-ordering service",
        "name": "v2",
        "version": "2.0.0"
      }
    }
  }
}

Note that the "versions" property is written like "bridge:versions". Why's
that? Because if I simply remove from the Context field "versions" type
(remove the line "@type": "bridge:dictionary"), then the Compacted document
is generated as expected, with "versions" property and without "bridge"
prefix.

I wonder what kind of logic is behind this.

Thanks in advance

Vagif Abilov