Axle measurement / Track adjustment Front axle afterwards Rear axle adjusted?

Hello together, I had the track set (front axle). This was a bit set. In the protocol I noticed that the track of the rear axle (left) before setting was within the tolerance ranges and after setting was no longer. The workshop informed me on demand that when setting the front axle the values of the rear axle could change automatically without that on the rear axle something was set and I would have in this case the one point I was not aware of that otherwise I would have commissioned it in this way, especially since it should have cost only 10 € more. What interests me: Why do the values of the rear axle adjust when the front axle is set? I thought the rear axle is the trailing axle and then the front axle is aligned. In case the question arises: With my car the rear axle can also be set. Thank you in advance.