I completely agree that there will be a margin of error, it's just about how to minimise this, and I still think it's much better than a person trying to work it all out in real time while running up and down.
I think they call it semi-automatic, because the VAR team have to look at it before telling the ref. I would guess that's partly to reassure some who don't trust tech, and just in case there's some kind of crazy anomaly, like when the goal decision system missed a goal a while back.
There's an article here, and the video in particular, while not very techy or detailed, does explains it a little more.
FIFA has announced that semi-automated offside technology will be used at the FIFA World Cup 2022™ in Qatar starting on 21 November.
www.fifa.com