The game modes available in PvP are Classic, Distance, Windy, and Wind & Distance. These modes generally affect how you aim. The current mode that is available to all players cycles every 15 minutes and the time remaining is indicated above the current game mode shown in the top-right of the PvP screen.
Classic mode makes it so Distance & Wind have no effect on aiming, so the bullet will land at the centre of your crosshair.
Distance makes it so only gravity affects your aiming. You will need to compensate for gravity by aiming above your opponent with your shots.
Windy makes it so only wind affects your aiming. You will need to compensate for the horizontal wind speed with your shots.
Finally, Wind & Distance makes it so both gravity and the wind affect your aim. You will need to compensate for both of these factors.