was the hillside you were on exactly perpendicular to the target, or was it facing in a direction slightly left or right of perpendicular? Were you lying prone exactly level with the horizon, or because of the slope to your left or right, were you slightly rolled a degree or two to the left or right? If the latter is the case, then perhaps it's an engine peculiarity. What if gravity is not modeled in the game? What if the game simulates gravity by deviating the bullet's path in the direction of 6 o'clock, or "down" towards the ground. I guess the theory could be tested if you could roll to the left or right, and fire down-range while on your back. If you could fire from on your back, and my theory is correct, your shot would curve upwards into the sky. Maybe this is why your shot seems to slightly curve left or right, when shooting on a hillside, but is straight when shooting prone from level ground?