Why "Curl -S" and "Curl" Give the Same Result?

On the command line, if I type

curl -s 

or

curl 

I get the same result:

{
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.63.0",
    "X-Amzn-Trace-Id": "Root=1-5eb2c764-3e67fbd6113ee4c0e5bccd8a"
  },
  "origin": "109.11.151.178",
  "url": ""
}

But if I use a pipeline to use the result of curl, for example, I type this:

curl  | grep -E '\d+'

Then I get:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   255  100   255    0     0   1237      0 --:--:-- --:--:-- --:--:--  1231
    "User-Agent": "curl/7.63.0",
    "X-Amzn-Trace-Id": "Root=1-5eb2c935-af183e03612a957bc5d1b48b"
  "origin": "109.11.151.178",

If I used the "-s" option for curl, the result of the pipeline is different:

    "User-Agent": "curl/7.63.0",
    "X-Amzn-Trace-Id": "Root=1-5eb2c990-5b0d5bb00b3c8aff7acf8d79"
  "origin": "109.11.151.178",

After some serach on the Internet, I got to know that "-s" option of curl won't print out the progress meter:

Silent or quiet mode. Don't show progress meter or error messages. Makes Curl mute. It will still output the data you ask for, potentially even to the termi- nal/stdout unless you redirect it.

• But where does the "progress meter" come from ?
• Why the "progress meter" doesn't show when I use "curl" or "curl -s" ?
• Why does it occur when a pipeline is used ?