I am glad that you do not just accept the answer, but try to understand it.
I know the formula, because I have read about that in a SAP note some time ago. Luckily I found it easily - 791944 - How is the KBETR determined in a subtotal?
What you can do about that?
Probably you cannot force the customer to buy in multiples of 10 kg and the business will not accept to define price 0.91/1 kg (with precision of 1 kg and your current prices you will always round) and I guess you also have percentage-based discounts, so you cannot avoid that completely even if your prices were defined per 1 kg anyway.
In general you can capture rounding differences and show them separately.