  A long-standing puzzle is why there is a difference between the optimal
doping \delta_{optimal}=0.15 for the maximal superconducting (SC) transition
temperature Tc and the critical doping \delta_{critical}=0.19 for the highest
superfluid density \rho_s in cuprate superconductors? This puzzle is calling
for an explanation. Within the kinetic energy driven SC mechanism, it is shown
that except the quasiparticle coherence, \rho_s is dominated by the bare pair
gap, while Tc is set by the effective pair gap. By calculation of the ratio of
the effective and the bare pair gaps, it is shown that the coupling strength
decreases with increasing doping. This doping dependence of the coupling
strength induces a shift from the critical doping for the maximal value of the
bare pair gap parameter to the optimal doping for the maximal value of the
effective pair gap parameter, which leads to a difference between the optimal
doping for the maximal Tc and the critical doping for the highest \rho_s.
