Every item in the game has an 'accuracy' figure, it's a measure of how un-aerodynamic the object itself is. Arrows and javelins have the lowest number (1), whereas plants and articles of clothing can have numbers in the hundreds. Items with lower numbers are more accurate when used as a missile. So if, in desperation, you were to throw your actual bow at an opponent, you could make a more precise throw with a Northern Bow ('accuracy' 5) than a heavy crossbow ('accuracy' 10).
To be clear, unless you are throwing the bow itself, this 'accuracy' value has no effect.
Your weapon skill, and the item quality (of both bow and arrow) all appear to contribute to tightening the cone of spread, with poor quality items making it especially hard to shoot straight. There is also a velocity calculation which is determined by the square of the bow's piercing/point stat, plus a constant (which appears to be the same for all bows). Higher powered bows (such as the longbow and Northern bow) will shoot higher velocity missiles, resulting in more damage on impact, and reducing the time a target has to get out of the way.
I presume, but have not confirmed, that higher velocity missiles also means missed shots will travel father, and are more likely to break on impact.
I have not checked to see if a bow's weight has any relevance on missile velocity. My gut feeling says it does not, but I do not have evidence to support that. Nor have I checked to see if the bows have different shooting speeds.