I’ve seen this question asked online for years and decided to offer my own explanation. I hope this helps.
The lighter blue line above signifies your lower control arm in its stock location.
When you add a spacer (signified by the orange line) it pushes the control arm down lower (signified by the darker blue line). As you can see, the space between the original lower ball joint location and the new lower ball joint location is greater than the space between where the strut mount location is on the lower control arm compared to where it as after the strut spacer. The further down the strut pushes on the lower control arm the more it’s magnified at the end of the control arm where the ball joint is (where the wheel would be). There for a 1-inch spacer is going to give you more than a 1-inch lift.
This isn’t the best drawing, but I hope it helps you understand.