Steel is different - my own 1.8m support rails dipped by 1.5-2mm in the centre, which is why I used epoxy - but in practice, how accurate are these aluminum extrusions?
I have built several router type machines with the heavy duty profile 90x45 with the L type gantry layout with the rails top and bottom with no issues. So in my conclusion they are fine for the majority of diy builds and work very well.