A photon counter counts photons less often at low intensity because they are arriving less often.Yes, it can be explained this way and that. And the sensitivity curves of the photon detectors speak also for the resonance. They look like the damped resonance curve (figure below).
So, the metal ion has a finite size and the electron is infinitely smaller.Yet you say they are about the same size.The metal ion is immobile. The electron, on the other hand, is very fast. In its chaotic movement around the metal ion, it blurs into a sphere.