Causes of Missing
Essentially every RPG has attack rolls which can miss, and GURPS is no exception, but there is rarely a lot of explanation about the the reasons people miss. There are a lot of them, and this analysis isn't even trying to handle melee weapons, but ranged weapons are somewhat easier to analyze. You can basically think of ranged combat as 'pick where you want to hit, and then a series of errors cause you to not actually hit there'. The basic causes for missing are:
The target isn't where you think it is. This can usually be interpreted as an automatic miss, though wild shots do occasionally hit someone.
The target is no longer where it was when you decided to fire. It takes around 200 ms for visual information to go from eyes to hands, and then some additional time for projectile to travel from the shooter to the target, during which time the target might have moved.
Sensor Limitations. Human vision is limited to about 1 minute of arc, so it's not really possible to aim more accurately than that.
Calculation error. For a bullet with a non-zero time of flight, you have to compute time of flight (equal to distance over velocity), then lead the target by (time of flight)*(perpendicular velocity), and aim high by (time of flight^2)*4.9 meters. While these are trivial calculations for a computer, humans are not so fast at math.
Error estimating range/time of flight. If you incorrectly determine the range, this gives an error of (perpendicular error)*(time error) + (time of flight)*(time error)*9.8 meters. Note that parallax detection (binocular vision) has a maximum range of (pupillary distance)/(angular resolution), and an error of roughly (actual range^2/max range); for normal human vision that works out to a maximum range of around 200 meters. Estimating based on target apparent size has a lower limit on error equal to the uncertainty in target size. At extreme ranges humans estimate ranges based on atmospheric haze, but that's extremely unreliable as it depends on knowing actual haze levels. In all cases real human performance is probably lower than those maxima.
Out of date information. If the target moves in an unexpected manner during that time
Error estimating target velocity. If you incorrectly calculate target velocity, you will miss by (time of flight)*(velocity error). Velocity error has a lower limit of (sensor resolution)/(observation time), which simplifies to (sensor resolution) * (time of flight / observation time).
Pointing Error. Normal human vision has a maximum resolution of about one minute of arc, and the physics of iron sights, where only one of the near sight, far sight, and target can be in focus at the same time, practically limits accuracy several minutes of arc, or less if the distance between near and far sight is short compared to the distance between your eye and the near sight. It also takes a reasonable amount of time to precisely aim, and if your hand shakes you might never reach that limit. Unaimed 'hip' shooting is dependent on your proprioception, which has much lower accuracy (probably a degree or worse). Throwing weapons are generally incapable of sighted shooting.
Mechanical error. Bullets don't always go exactly where you aim, due to mechanical