We see from the three vertical axes on the figures that the smaller l becomes with respect to d, i.e. the smaller the ratio l/d which is given in the figures, the smaller the effect of the quadrupole terms become. Indeed, for l/d = 0.6 the quadrupole term causes the graph to achieve values of lower than -1, for l/d = 0.2 we have around -0.83 as most negative y value, and for l/d = 0.05 we have -0.808 as lowest y value.