Assuming this is D&D, More then likely this is due to advantage/critical strike settings on the character sheet. Usually there are some options for these, but the default is usually to show 2d20 on the attack roll, then if you have advantage/disadvantage you pick the higher/lower. if not, you pick the roll on the left. I believe the same thing happens for critical damage as well, but i'm not 100% sure on that one.