Wrong dimensions in map obtained using GMapping?

Hi all, I have a map which I obtained using GMapping and now I am using it for navigation. The robot gets lost when the laser scan particles do not fall directly on the same wall on the map. In the image shown there is a difference of 1.32m between the actual wall and the wall on the map. The dimensions of the map do not always match reality. Sometimes there are discrepancies of around 1m between the dimensions shown on the map and reality. Does anyone have any ideas why this is happening?  Thanks.