Training. No scope (or hard sight) is 100% accurate at all ranges, because bullets don't travel in a straight line. Gravity will always have an effect. Bullets actually travel in an arc. Usually leaving the barrel (most notably in long guns/rifles) at a slightly upwards angle, and then start falling after a bit. Like this --

https://ke-courses-production.s3.amazonaws.com/asset_files/production/3766/attachments/original/muzzleloader-trajectory.jpg

And experienced/trained marksman (hunter, sniper, etc) knows how to compensate for this at various ranges