  Recent mass measurements show a substantial weakening of the binding energy
difference delta_{2p} (Z,N) = E(Z-2,N) - 2 E(Z,N) + E(Z+2,N) in the
neutron-deficient Pb isotopes. As delta_{2p} is often attributed to the size of
the proton magic gap, it might be speculated that the reduction in delta_{2p}
is related to a weakening of the spherical Z=82 shell. We demonstrate that the
observed trend is described quantitatively by self-consistent mean-field models
in terms of deformed ground states of Hg and Po isotopes.
