The measurement of sound absorption in reverberation rooms following the ISO 354:2003 standard relies on Sabines equation to derive absorption coefficients from reverberation times. This equation assumes perfect diffusivity, i.e. the sound field is composed of many statistically independent plane waves with uniformly distributed spatial phases, themselves uncorrelated to the corresponding amplitudes. In this work, both existing and fictitious reverberation rooms are numerically modelled using the finite element method. Finite porous absorbers are introduced in the rooms as equivalent fluid models. Standardized sound absorption measurement are simulated in the rooms through the determination of reverberation times. The respective effects of the sample size, sample placement, source positioning, and presence of finite panel diffusers are investigated. The resulting absorption coefficients are then confronted to the theoretical values in a perfectly diffuse sound field, that interacts with a baffled, finite-sized absorber, as obtained with a hybrid deterministic-statistical energy analysis model. The process notably underlines the strong, yet often disregarded, beneficial effect of panel diffusers at low frequencies in highly regularly-shaped rooms. Another conclusion of this work is that reverberation room design represents a crucial factor that can influence sound absorption measurements at low frequencies.