Bizarre baggage weight policies

Checked in 6 bags with British airways and had to pay for 3 bags that were overweight. Now how do you explain that one bag is 1 kg overweight and must be paid for, but the next bag is 6kg underweight, and no consideration is given to this? On average, our baggage was correct weight, but because some were over and some were under, I paid ******ionate penalties! Shocking, thieving policy! Not to mention the unhelpful lady at the check in!