Hmmm: 'I was sure it was offside' - why did Wirtz equaliser stand?
I wasn't aware of this 'tolerance' rule (and I don't seem to recall any other examples), but apparently it's been a thing for while, which seems to add an extra level of complexity to the 'clear and obvious' rule.
Different systems, though.
Prior to this season it was some pleb in the VAR room drawing a line on the pitch. The thickness of the line added a bit more tolerance.
This season, it uses automated systems. Where even the slightest body part in an offside position shows.