VAR officials have been told to look out for certain incidents at the World Cup following criticism about unpunished off-the-ball shirt-pulling.
England's Harry Kane and Serbia's Aleksandar Mitrovic both appeared to be held in the box in group-stage matches.
"You might have appreciated there were some incidents that suddenly disappeared," said referees committee chairman Pierluigi Collina.
"It's impossible to be right from the start. We intervened and fine-tuned."
A total of 335 incidents were checked during the group stage - nearly seven per game - with 14 on-field reviews made by referees and three reviews made by the VAR team on factual decisions.
Referees called 95% of incidents correctly without VAR, but the system - which is being used for the first time at a major international tournament at Russia 2018 - improved that success rate to 99.3%.
"We have always said that VAR doesn't mean perfection - there could still be the wrong interpretation or a mistake - but I think you would agree that 99.3% is very close to perfection," added Collina.