Using a large sample of sub-L-* galaxies, with similar UV magnitudes, M-UV similar or equal to -19 at z similar or equal to 6, extracted from the FirstLight simulations, we show the diversity of galaxies at the end of the reionization epoch. We find a factor similar to 40 variation in the specific star formation rate (sSFR). This drives a similar to 1 dex range in equivalent width of the [O III]lambda 5007 line. Variations in nebular metallicity and ionization parameter within H II regions lead to a scatter in the equivalent widths and [O III]/H alpha line ratio at a fixed sSFR. [O III]-bright ([O III]/H alpha>1) emitters have higher ionization parameters and/or higher metallicities than H alpha-bright ([O III]/H alpha