Very possible that line WAS drawn in ArchiCAD on the able, mentioned in another CAD. Just because it's 89.7... and angle dimensions are set to show only numbers before point - ArchiCAD is rounding it to 90 degrees.
Try to draw your lines holding Shift key - you will get strict 90, 180 and 270 degrees. Also switch on two decimals in angle dimensions and working units - then you will get better picture on the angle.