Their gross margin is 56%, their net margin is around 10-12% that is very good for FMCG business
they can improve more if they control cost.
the margin is there for the taking if they good with cost control it will rake in
if you go by your analysis you would never invest in Woolies or Wesfarmers two stocks that reward investors for decades in capital gain and dividend its net margin is around 5-8%