its just giving you warning that fuel is low - and when fuel light flashes there is about 6 to 7 litres in tank, rather have that than be warned seconds before the engine cuts out at night on a rainy motorway. Some people are very good at watching fuel gauge and topping up tank, but some are not, also in some countries garages are far apart - every car maker has to build cars for the world now, you will probably find most cars are same now with early warning.
as far as second half of tank going down fast, the full to half full indication on gauge is actually half a tank (21 litres) but the second half is not half but 21 minus 7 litres as the car considers tank to be empty when it gets down to 7 litres, this is why 'last 100 miles goes down quick'.