To begin with, it is important to remember that the GPS unit only reports positions every (depending on the product) 120/30/15 seconds. When you look at the history, you must therefore bear in mind that the movements that take place between updates are not logged. Therefore, it can sometimes appear as if the vehicle has "taken the straight line". You should also keep in mind that all GPS receivers have a certain margin of error, sometimes up to 15 meters, depending on the coverage conditions where the product is used.
If you experience a consistently poor precision of the location, it may have to do with the signal strength. Initially try to adjust the placement, you can also read the following article for general suggestions on how to improve the accuracy.