That really depends on the car. I don't own a car so often use rental cars. Most cars' consumption is within 10% of the shown values but at least every fifth car I drive has vastly different numbers.

I think it is not necessarily due to the car but because of the calculation methods they use. If you drive motorway and city, consumption is accurate. But as soon as you start driving in the mountains or a mix of different terrains, data often vastly differs.