To expand on this for those who don't follow soccer: If the AR pops the flag for offside, that stops the play - and if he is wrong, it may cost a team a great goal-scoring opportunity. His mistake could easily decide the game. But if he mistakenly does not flag for offside, allowing play to continue, and a goal is scored, then VAR will disallow the goal and no harm has been done. So it makes sense to not put up the flag unless you are sure - since one kind of mistake can be catastrophic while the other kind of mistake causes no harm (other than to trigger a brief delay).