Ok, apologies for taking this apart but as someone who has worked in this industry offside is not 100% black and white using technology. By the same notion Goal Line Technology is 100% black and white but in reality it has an error of +-2.5cm.
Every technology has measurement errors. For offsides the two main ones are:
1. Triangulating co-ordinates using multiple cameras to find the players locations and draw the offside lines. Especially when the lines are drawn manually and significantly impacted by pixel resolution.
2. Determining the point the ball has been kicked and when the player is offside. The 50fps cameras used for this are not fast enough and don't give the resolution to determine this accurately.
It is perfectly sensible, and I would argue necessary to have a margin for error.