为何jq过滤器'exp as $var | .'未按文档输出原始输入？

官方文档说明 #

jq官方文档指出（重点标注）：

表达式exp as $x | ...的含义是：针对表达式exp的每个值，使用完整的原始输入执行管道剩余部分，并将$x设为该值。

我的疑问与测试示例 #

我原本理解：在exp as $x | ...结构中，...的执行上下文是以jq命令的原始输入为起点。但实际测试时，exp as $x | .并没有只输出一次原始输入，而是重复输出了多次，以下是具体测试内容：

使用的过滤器 #

(.signal[] | select(.name | contains("signal"))) as $matches | .

输入JSON #

{
  "noise1": 5,
  "signal": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ],
  "noise2": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ]
}

实际输出结果 #

{
  "noise1": 5,
  "signal": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ],
  "noise2": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ]
}
{
  "noise1": 5,
  "signal": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ],
  "noise2": [
    {
      "name": "child-signal-1",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-noise-2",
      "nested": {
        "prop": "child-prop-3"
      }
    },
    {
      "name": "child-signal-3",
      "nested": {
        "prop": "child-prop-3"
      }
    }
  ]
}

核心疑问 #

1. 为什么exp as $x | .的输出是重复两次的原始输入，而非单次输出？
2. 明明管道右侧没有引用$matches变量，为什么exp的结果数量会直接影响最终输出的次数？

内容的提问来源于stack exchange，提问作者Daniel Kaplan