How is the baggage allowance when checking in multiple pieces supposed to be calculated? Cumulatively across the pieces or with individual pieces rounded up?

I just checked-in at Johannesburg with 3 bags. The weights of the bags were :

13.6kg
17.5kg
14.7kg

By my calculations, this meant that my total baggage used was 45.8kg, which was within my allowance of 46kg (30kg standard allowance + 16kg extra allowance for Skywards Gold).

The check-in girl however said that this came up to 47kg and therefore I was overweight by 1kg and would have to pay R210 in excess baggage charges. She was rounding each individual piece up and then adding them together, so she saw this as 14kg, 18kg and 15kg.

Supervisor was called and no excess baggage fees were charged in the end, but I was wondering if anyone else had a similar experience in the past and what the official policy may be?