VAR review: Why was Wirtz onside in Premier League, offside in Europe?

Liverpool had a goal from Florian Wirtz ruled out vs. Fulham, but it was overturned by VAR on the slimmest of margins. Why?

By Andy Davies, Special to ESPN · ESPN Top Headlines

Liverpool had a goal from Florian Wirtz ruled out vs. Fulham, but it was overturned by VAR on the slimmest of margins. Why?