The RPGs are nicknamed "wobbling goblins" apparently.
The initial charge expels the round from the tube, and then some large knife-like fins pop out to stabilise it. Unfortunately, these fins make the round very susceptible to weather-cocking (the nose of the round will tend to point into the wind).
Once the round is out and attempting to stabilise, a secondary rocket motor fires to give it the range. The rocket will kick in at a random orientation of the wobbling round - so where it goes next is a little bit indeterminate. This secondary charge initiation is apparently the main source of inaccuracy.
Apparently some folks are "natural" RPG gunners, a bit like with cricket bowling or baseball pitching. They are few and far between. The system also needs a lot of rounds fired by the gunner to achieve any sort of proficiency.
Tube AT weapons generally "spin up" the round inside the launcher - rifled ones (Carl Gustav) certainly do. Western rounds tend not to have a secondary "kicker" going off down range to disturb the initial trajectory.
Secondary rocket boosters are always a problem - e.g. unguided RAP artillery rounds, which have a longer range,
but also a wider dispersal of impacts due to the rocket initiation being semi-random throwing the rounds off course, depending exactly when they kick in.
All you wanted to know about the RPGs:
http://www.smallarmsreview.com/pdf/RPGDec06.pdf
Cheers
Andy