Not so much the spring rate changing, but you are changing the loading on the front and rear halves of the spring. It is like putting a bottle jack in between the frame and rear spring eye (with the shackle removed). Start lifting and you have the frame raising but also you have the rear of the spring bending down. How much of each happens affects the lift.

And with a longer shackle, you may also have the factor of shackle angle. A shorter shackle may be kicked back farther while the longer shackle stands up more vertical, so you would see more lift than the simple length change. Basically just a lot of variables involved. That is why I tell folks I can make the a shackle X" longer than stock but I can't make them a shackle for Y" of lift.