Depends on the machine, the offset could mean the small angle between the sample surface and the preferable plane of your sample. Please provide more information like what machine you are using, how you perform the sample alignment (for XRR or other?).

I use the Rigaku SmartLab. There's an XRR package using PB medium-resolution optics and I used its sample alignment. I define medium density, and standard alignment leads to the same result as precise. 

Thank you for your guidance. 

2theta offset is related to a misalignment of the detector. Therefore it is a simple constant offset. There are several factors which interfere but sample alignment and 2theta offset have nothing to do with each other.