That explains so much. When Wizzair made their €5 cash grab to bring their baggage allowance from 15kg to 20kg (which it was at before this extra charge), I thought they were on to something that FR hadn't seen yet. I guess they've instead just skipped that cash grab and declared the allowance in 15kg increments.
Looking forward to pay-per-kg rates, even if I'll hopefully never experience them again.