Why do i have to pay excess luggage charge when my total weight incl.luggage is less than the next guy?ee?

i weigh 60 kg,i had 40 kgs of luggage and had to pay exrtra. There were pengers weighing 150 kgs who pay no excess,despite ,in total,puting more onto the plane than me.Is this fair? Surely the charge should be based on the total weight you are putting on the plane,not single out the luggage.