We identify the spectral gain filtering as a mechanism of multiple pulse formation in a fiber laser passively mode-locked through nonlinear polarization rotation operating in the normal dispersion regime. A correlation is theoretically established between the spectral gain bandwidth and the possibility for the laser to deliver several pulses by cavity round-trip: narrow spectrum favours multiple pulsing. Moreover the model allows to show that spectral loss or gain filtering are equivalent with respect to their consequences on the multiple pulsing regime. Experimental results obtained with an erbium-doped fiber laser and an ytterbium-doped fiber laser confirm the theoretical calculations. In addition, an indirect confirmation of the influence of the spectral filtering is proposed using an experimental setup involving a unique laser.