[DoctorOetker]

Quantum Efficiency is defined as the number of electron hole pairs per photon.

Without avalanche effect, without quantum cutting phosphors etc, the quantum efficiency for visible light photons can not exceed 100%: a photon of suitable energy (i.e. higher than the bandgap) would liberate a single electron-hole pair with the excess energy lost as heat. Some electron hole pairs may recombine before completing a loop around the external circuit, lowering the total quantum efficiency.

A SPAD uses the avalanche effect: the applied reverse bias being so strong that the electron of the initial electron-hole pair is accelerated to sufficient energy to knock out and liberate more electron hole pairs, so that you can have quantum efficiencies exceeding 100%: as a function of the applied reverse bias there is an extra mean multiplier constant describing how many electron-hole pairs will be liberated due to a single photon striking.

>This explanation isn't consistent with my understanding.

Its probably because you misremember, the quantum efficiency of SPAD's is higher not lower than that of your average CCD sensor.