We investigate the dependence of the yield of superheavy nuclei with Z=110, 112, 114 on the neutron excess of the projectile nucleus with a two-parameter Smoluchowski equation. It is confirmed that in some cases, the cold fusion reactions with less neutron excess are more favourable than those with more neutron excess. In order to probe the origin of these unexpected isotopic trends, we also investigate the probabilities of capture, fusion and survival in the cold fusion reactions in detail. It is found that the maximal ER cross sections of the superheavy nuclei exponentially increase as a function of B_f-S_n with B_f being the fission barrier and S_n being the neutron separation energy. Although the probabilities of capture and fusion have some influences, the unexpected isotopic trends mainly due to the ependence of the ER cross sections on the B_f-S_n value. Therefore, the reactions with larger B_f-S_n values should be more favourable for synthesis of superheavy nuclei.