Maximum unit length on site

Why do the CC go to the trouble to list a maximum unit length on a site when in reality this is ignored by wardens and members. Yes I can understand site access and depth of pitches as being limiting factors, but if that is the case then the limitation should be observed. For instance on a recent visit to a site which has a maximum 8 metre unit length, parked near to us was a 8.56 metre motor caravan. This is not the first time that we have seen this "rule" ignored. Surely if the CC has quoted a max unit length there ought to be a logical reason for it and it should be observed otherwise why bother producing this information.