From the Advanced Bash Programming Guide:

"In general, an external command in a script forks off a subprocess, whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents."

And a little further down:

"A command list embedded between parentheses runs as a subshell."

Examples:

[root@talara test]# echo $BASHPID
10792
[root@talara test]# (echo $BASHPID)
4087
[root@talara test]# (echo $BASHPID)
4088
[root@talara test]# (echo $BASHPID)
4089

I believe that explains the rule as you requested.

From the Advanced Bash Programming Guide:

"In general, an external command in a script forks off a subprocess, whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents."

And a little further down:

"A command list embedded between parentheses runs as a subshell."

Examples:

[root@talara test]# echo $BASHPID
10792
[root@talara test]# (echo $BASHPID)
4087
[root@talara test]# (echo $BASHPID)
4088
[root@talara test]# (echo $BASHPID)
4089

Example using OPs code (with shorter sleeps because I am impatient):

echo $BASHPID

sleep 2
(
    echo $BASHPID
    sleep 2
    echo $BASHPID
)

The output:

[root@talara test]# bash sub_bash
6606
6608
6608