Our nominal finished SPF 2x4 is precisely 1.5" thick.
With the radiused edges, you can stack two pieces for a window sill, door frame or wall that comes out to 3".
Then you add a stud (92 5/8") plus a cap for a 96" wall frame. I know the math doesn't add up. But it works.
Now, if the 2x4's had very square planed corners, it's very difficult to pack them to see just 3". Why that is, I don't understand.
In a past life, I framed and finished a 1200 sq ft concrete basement. Quite a learning experience just to get it framed!