Why does command substitution change how quoted arguments work?

Question

I have the following snippet:

printf "%s\n%s\n%s" $(echo "'hello world' world")

I would expect it to produce:

hello world
world

But it actually produces:

'hello
world'
world

Why is the above command not the same as the following?

printf "%s\n%s\n%s" 'hello world' world

Answer 1

After command substitution, only word splitting and wildcard expansion are done, but not quote processing.

From tbe Bash Reference Manual

The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.

The description of word splitting is:

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into words on these characters. If IFS is unset, or its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and <newline> at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space and tab are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs.

No mention is made of treating quotes specially when doing this.