Rectangle shrinks symmetrically, expands to a side

[mstas227]

As you can see here, two consecutive similar actions result in different behaviour. Moreover, changing width in the lower rectangle works correct. Both rectangles are copies. Is there some basic knowledge or steps I am missing here?

Sorry, cannot upload video, but basically it shows that when you first reduce size of the width of the upper rectangle from 800 to 650, it shrinks simmetrically to the origin. Then when you make it 800 again, it will expand only to the right side, assymetrically to the origin.

[SCALEMODEL]

You can simplify your constraint and not use construction line. Just use symmetry constraints on your 4 rectangle, I believe that would also accomplish what you are looking to do.