By fixing a parameter for the measured mean like it was done in 1) one has to apply a correction factor. If the true mean is known this is not neccessary any more.
$\text{cov}(N,N') = \langle N N' \rangle - \langle N \rangle\langle N'\rangle = \langle ( N' + \delta N ) N' \rangle + \langle N' - \delta N \rangle \langle N'\rangle = \langle N'^{2} \rangle + \langle \delta N \rangle \langle N' \rangle - \langle N' \rangle^{2} - \langle \delta N \rangle \langle N' \rangle = \langle N'^{2} \rangle - \langle N' \rangle^{2} \approx N'$
$\text{cov}(N,N') = \langle N N' \rangle - \langle N \rangle\langle N'\rangle = \langle ( N' + \delta N ) N' \rangle + \langle N' - \delta N \rangle \langle N'\rangle = \langle N'^{2} \rangle + \langle \delta N \rangle \langle N' \rangle - \langle N' \rangle^{2} - \langle \delta N \rangle \langle N' \rangle = \langle N'^{2} \rangle - \langle N' \rangle^{2} \approx N'$