Are r.t. trip tickets ever cheaper?

married and was going to ask this there but i decided here, my husband worked in kuwait and cheated on me with a manipulative, greedy skank philipina, well he came home, i forgave, but i noticed his plane ticket was round trip, he said because the r.t. was cheaper than a one way, is that true? i doubt it, arent you paying for the fuel why would it be cheaper.