Distance trace malfunction

There are increasing amounts of vehicles which have a distance trace similar to the one opposite, where the trace, on occasions, does not travel to the full length to the bottom.  There are four dotted lines in the distance trace section of the chart and under normal circumstances the distance trace should travel from the very top line to the very bottom line.  However is some cases the trace stops short, in this example at the next to the bottom line and in other cases much higher.  If anyone has any knowledge as to why this happens could they please let us know here.