(T) Fuel tank, confused!!

hi people, after a visit to my local fuel station and i leave with some question marks in my head.
Ok, all bravo comes with a 58L tank, I was running very low today, after the warning light comes on I done another 60miles on it, when I stop at the fuel station, computer tells me i have a range of 31miles left in my tank. However, after putting 42L of fuel in my car, it was full, I try to put as much as I can, 42.8L is what I can managed!! so, am I right I still got at least 10L of diesel in my tank before I fill up? in this case, why the warning light comes on and the computer tells me I only have 31miles worth of fuel left?
The car ave about 45-48MPG with a very heavy right foot, which I think is very good.

Every car has a 'reserve' amount of fuel. This means that when the car is registering as 'empty' with a range of 0 miles there is still fuel in the tank.

On the petrol version I have the reserve is apparently 7 litre IIRC? I assume the Diesels would be the same?

This is a built in safety feature to stop people running out of fuel. It's amazing the amount of people who won't even think of filling up with fuel until the dial reads zero or empty. And even then some people think it's some sort of achievement to keep going lol

I've had exactly the same with my 150mj sport. Most i've ever got in the tank in one fill was 51 litres and that was about 50 miles after the fuel light came on. I did fill up the other week and seemed to get a lock somewhere in the filler pipe - 42 litres went in, less than I was expecting, but the fuel gauge didn't make it to full - I'm guessing it stopped about 6-7 litres short of a full tank.