Gwandad, the 53 lts is the nominated capacity, but as some have explained, the fuel tank is "amidships" and so there is a long neck, so that if you fill to the brim, let it settle and fill again, you can fit more in.
The Maximum I have done is 58 lts but Rustynuts got just on 59 one time.
But, I also had the same thought as you....how do we know that the bowser is metering it correctly..

As for the amount left when the light comes on... I am still not sure. Once my ECU data reader arrives I hope to have a better handle on all this.
Cheers
Elle B