The weight for a given volume of Jet A changes with temperature. At 15.6C Jet A-1 is 1.78lbs/l, however if the temperature were to rise above this the fuel would expand. In this case 1/l of fuel is going to weigh less than 1.78lbs/l.
This is called the specific gravity of the fuel. It is a measurement taken whenever needed to let the flight crew know how much their fuel weighs. If for example an aircraft took on 40,000L of fuel at 1.6390lbs/l the total weight of fuel is 65,560lbs. If the SG for the fuel was 1.8040lbs/l the total fuel weight would be 72,160lbs. That's a difference of 6600lbs.
This is important for crews when calculating balance fields, or V speeds etc. Hope this answers your question.