Change basic cube to a board shape, Z=.1, X=8, Y=.4. Edit mode; grab back left 2 vertices along X axis for a 60 degree angle on the left side of the board. Select all vertices, and do Mesh - Symmetrize - -X to +X. Now the other side looks great, too. Duplicate object. Scale smaller only on the X axis. Results is the 60 degree angle is now less than 60 degrees.
Why? Why didn't it make a shorter board with 60 degree angles on each end? Why did it not scale the back vertices the same distance as the front vertices? Is there a way to make it scale both the back and front vertices at the same time the same distance along the X, (or any), axis?
I tried "Delta Scaling" and got the same results. It seems to apply a percentage of distance, instead of the exact distances I provide it, thus distorting the shape.In other words, (and using different figures ... not the ones above), a 1 meter box with the front left at 1 meter, but the back changed to .8 meters, when scaled from 1 meter to .8 meters will take 20% off the length of the front, and 20% off the length of the back, instead of taking .2 meters off of both, as expected.
These angles don't change when the entire object is scaled, (vs. just one axis), using the Scale command, but it does when entering the exact values into the Transform tab, where one axis at a time has to be entered. Why would it do it differently for a single axis?
It was wrong of me to expect someone to build this to see what I mean, so I made some screen shots: