How to understand the following quote from termios description

Question

I'm using Mac (10.14.6), following quote is from man termios:

When a controlling terminal becomes associated with a session, its foreground process group is set to the process group of the session leader.

To verify this, I opened a terminal window, ran sleep command in the foreground, then opened another terminal windows and ran ps:

$ ps -o pid,pgid,tpgid,sess,stat,command,tty
  PID  PGID TPGID   SESS STAT COMMAND    TTY
44606 44606 45006      0 S    -bash      ttys000 
45006 45006 45006      0 S+   sleep 3000 ttys000  

which shows foreground process group as 45006, to get the session leader I wrote some C code using getsid and getpgid, then get following info:

pid: 45006 pgid: 45006 sid: 44605
pid: 44605 pgid: 44605 sid: 44605

the session leader is 44605 which is the login proc:

$ ps -o pid,pgid,tpgid,sess,stat,command,tty -p 44605
  PID  PGID TPGID   SESS STAT COMMAND          TTY
44605 44605 45006      0 Ss   login -pfl mz /b ttys000

obviously the foreground proc group 45006 differs from session leader proc group 44605, what am I missing? Thanks!