A land rover isn't just a normal car is it!!? If you consider the working conditions of a typical 'car' engine, you're looking at it sitting practically horizontally 80% of the time, with the other 20% of the time being slight inclines and descents on the motor-way and local hills for example, all with tarmac roads that are considered 'suitable for vehicles'. Most numpties out there only need to keep their oils levels just right for the majority of roads, so the manufacturers just stick an 'N' on the stick to indicate the normal level adequate for operating, leaving no room for arguments. A land rover engine on the other hand will typically alternate between sitting at anything from horizontal to near vertical on grassy slopes and fell tracks that aren't suitable for common road going vehicles, so the need for a more accurate oil gauge is necessary. Obviously if your descending a hill at a steep gradient in your N/A, the oil pump pickup (near to the back of the sump) is going to struggle to pump as much oil as usual because the laws of physics have the oil move to the front of the sump where only oil hats sloshes about towards the back of the sump can be picked up by the oil pump.
So to kind of simplify it a bit, if you're going off roading, you'd fill your engine oil to the 'H' mark, and not a drop over, so that there is adequate oil for the constant movement of oil in the sump in relation to the oil pump pick up pipe, but not too much oil that it should over run, where as if you were on the flat most of the time, you could run it to almost 'L' before you'd need to top it up a bit more.
It's just their way of thinking I guess.
-Pos