Even Wirtz had not expected the goal to stand, telling Sky Sports: “I was sure it was offside so I didn’t even celebrate.”
It was given, though, because positions are measured with a tolerance level of about 5cm (1.95in).
That was initially introduced to take into account inaccuries in the offside technology used several seasons ago – for example, being able to judge the correct moment the ball had been passed.
In effect, a player could be positioned marginally offside but ruled onside when the tolerance level was applied.
The Premier League retained that tolerance level when moving over to the semi-automated offside system now used.
Another element involved is the choice of frame. The video assistant referee (VAR) is offered three frames and chooses the first to show contact in the act of playing the ball. It is not when the ball leaves the foot.
In Wirtz’s case, the VAR chose one frame and TV broadcasters the next. Wirtz is more clearly offside in the second.