Every aircraft design is a compromise between conflicting factors - wing thickness versus structural strentgh, for example.
Derivatives like the 319, 318 and 321 result from changes to a design which has been carefully optimised, so it is likely that performance in some areas will improve at the expense of degradation in others.
A specific suggestion for the difference in range you mention: perhaps the shorter fuselage allows less stabiliser balancing arm, so the tail downforce has to be higher. This might account for it, but I don't know for sure.
The website shows pretty simplified data - my point is there are many other factors which influence performance apart from wing design and overall weight.