Fractional difference between the mean of simulation pairs at the fiducial cosmology (i.e. model-FID) and the input theoretical model P(k), obtained with HALOFIT. Faint blue dotted lines show the results for a number of random pairs at the initial redshift z_i = 120, while the thick blue line highlights the best pair. The sampling variance cancels to better than 5% also at z = 0.64 and 0.04, as demonstrated respectively by the red and black lines. The grey zone indicates the regime where the discrepancy exceeds 10%.