Go to My Documents > My Games > Skyrim and open the Skyrim.ini file. Add these two lines to the bottom:
[Actor]
fVisibleNavmeshMoveDist=12288.0000
This makes your arrows hit pretty much anything you can see, if you aim properly. Without it the game doesn't register arrow hits at large ranges. You will have to aim a pretty good distance above your target to hit stuff you can barely see. Practice on mammoths.
Sweet, I'm gonna have to do that.
As for the trajectory. It's called an arc. The game automatically arcs your shots for you to a small degree allowing you to simply point at center of mass for a greater distance without the need to compensate for gravity.
http://www.crossbowmen.com/trajectory-2.gif
With my compound I use 4 pins, starting at 20 yards and increasing by 10/pin. Any distance within 20 yards I just use my 20 yard pin and will hit within 2 inches anyway (accurate enough to hunt with which is all I use it for).
In Oblivion, the trajectory was straight and by that I mean that the arrow left heading towards the point of aim directly but began dropping immediately. The meant that you had to compensate for distance unless the enemy was basically within melee range.