You must be used to normal cars where the needle drops like a lead balloon.
Seriously though, there is a long pipe from the filler cap to the front mounted fuel tank on the Jazz (tank is under the front seats) and if you fill up completely this pipe has to empty before the tank level starts to drop, this can be quite a few miles. I have on occasions driven very carefully to see how long I can keep the needle on 'full', and have been surprised at the result.
Another feature of the Jazz, although this is common to other cars is the fact that the first half of the tank seems to last a lot longer than the second half, but if you take the reserve fuel (I think about 7 litres on the Jazz after the light comes on) it is logical.