A318s can have different engines (P+W6000 series) not used on the other models - might check to be sure the range listed is for the same engine (CFM65).
Some A318s are assigned artificially low MTOW limits (lower initial price, lower landing and ATC fees) - they may have tanks that hold 19000 kg, but never use full tanks, to stay below the MTOW. Some thus have a range as low as 1500 nm.