The answer may lie in the fact that the OS people over the years have been [how shall i put this] a little slack in many parts of the country [Cope for one has a few times gone on about how slack a lot of OS maps are], The 25000 OS map of Dartmoor is a good example because there's loads of ridges and even hills that aren't even shown or are just plain wrong, just don't put to much trust in maps because they're never 100% right, that's why google earth and the like are popular, because what you see is at least real, even though it's flat and the earth isn't.