Why actual fuel consumption in most of the cases differs from what automaker or central computer claims?

What is it – inevitable error or intentional skullduggery?

In reality, there is no global conspiracy on fuel use among car-makers. Everything is more or less simple – you cannot please everyone. Some people live in suburban Berlin, and usually stuck in traffic, others use their car only on highway. Someone lives in the mountains, while someone else is a taxi driver with all possible consequences. Someone travels alone, someone – only with passengers… We can continue forever.