  We present a detailed analysis of the physics governing the collection
efficiency and the Purcell enhancement of the nanopost single-photon source. We
show that a standard single-mode Fabry-P\'erot model is insufficient to
describe the device performance, which benefits significantly from scattering
from the fundamental mode to radiation modes. We show how the scattering
mechanism decouples the collection efficiency from the Purcell enhancement,
such that maximum collection efficiency is obtained off-resonance. Finally, we
discuss how this scattering mechanism can be beneficial for future
single-photon source designs.
