Everything @brvdboss said. Several people have milled their wasteboard at an angle because they incorrectly assumed that the X axis is always parallel. It is not, and can easily be tilted by raising and lowering the Z tower mounts where the X axis connects to each tower. If you are measuring the right side consistently lower than the left that that’s a very good indication your X axis is not tram to the linear modules that make up the Y axis.
MS-paint-cad exaggerated view here: