What causes Image Orientation Patient slightly off from standard unit vectors

[J Chui]

Hi, we have some application software for CT that requires ImageOrientationPatient tag to be axial, such as 1,0,0,0,1,0, with no more than 1 degree offset. In reality, many of the series I have seen are off by 2-4 degrees. For example, 0.9999 instead of 1. 
In reality, what is the major cause of these offsets? Is it something we can correct on the scanners?

Thanks, 
J Chui.