The arc is just a result of gravity.That is why when you shoot up a hill on a 45 deg angle,you shoot the shot the distance the target would be straight out from you instead of what it would range on the angle.

Gravity only affects the arrow for the time it is in the air and gravity doesn't see angles up hill or downhill.It just pulls STRAIGHT down on the arrow on its actuall horizontal path.

Hope this makes sense.