This one always confuses me I'm afraid. If I fill my car up when the low fuel light comes on I put aprox 45 litres (12.5 US gallons)of gas in, if I wait a bit I can easily put 55 litres in the tanks is 60 litres (16 US gallons) so for me there is no issue (this is in Canada) the tank should be the same so I think if you are putting gas in when the low fuel light comes on (which is when the car hits the "reserve mark) you will be putting in aprox 12.5 gallons (as it says in the owners manual) this has been Volvo's practice all along (from my 120 though every model (yes I have had at least one of each) to the V40 it is the way Volvo has always set up their fuel gauges. Now if your low fuel light isn't coming on at the right place on the guage (if you are way below the red area) then there is apparently a way to calibrate the fuel guage (it has been posted either here or on volvospy) so that it is working properly (I believe it was through software). In any case my car is proof that the tank will let you use more than 12 gallons(on one trip to the states last year I put about 15 gallons in when I couldn't find a gas station right away). In any case enjoy your car.
Scott