if you ever take apart a tension, you'll discover that you can move things around so the "numbers" are lined up differently.
They really mean nothing. You'd have to do a pull test to see if a setting of 4 on one machine compares to a setting of 4 on another.
My Juki does well with 2- but it only has 0-4. My Viking does well around 4, but it has 0-7. My HQ LA does not even have numbers, but the digital pressure gauge I installed has settings from 0 to . . . I'm not sure, I 've never gone over 800. Some thread works best in the 450-500 range and some best in closer to 700. And I've found moving just 15 points can make enough of a difference.
Come to think of it, very few of my older (75+ years) machines even have numbers for tension. I prefer numbers just so I know if I'm turning it the right way. I know "lefty loosey, righty tighty" but I still get confused as to which way is right and which way is left. So I need the numbers to take the "left/right" out of the equation.