Difference b/w Aggregate & joinaggregate in vega-lite

When I use aggregate i'm getting value as 0, but when I use joinaggregate i'm proper value.
Why is this happening ?


{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "width": 400,
      data: {
    url: {
      %context%: true
      index: index_dash_performance2
      body: {
        size:10000
        _source: [ "StatusOpen"]
        }
      }
      format: {property: "hits.hits"}
    },
  "transform": [
      {"filter": "datum._source.StatusOpen != null"},
      {
        "aggregate": [
        {"op": "sum", "field": "_source.StatusOpen","as": "aud_count"}
        ],
      }
    ],
  "layer": [
 {
    "mark": {
      "type": "text",
      "align": "left",
      "dx": -6,
      "dy": -5
    },
    "encoding": {
      "text": {"field": "aud_count", "type": "quantitative"}
    }
  }]
}

I think you might be working on the same spec as another Discuss user who I helped out: