As Guzzilazz has pointed out there are many things that will cause a mover to wander off course, anything that causes one wheel to have to travel further (e.g. a stone) will mean the caravan will not move as far forward on that side, It will also increase the load on the motor as it has to climb the obstruction, which will further slow it down. Whilst only a relatively small difference it will affect the track of the caravan.
As the motion is already quite slow, there is plenty of time to compensate for any terrain induced change of direction.
Really this aspect of caravan movers performance is really nothing to get overly concerned about.