Accounting Karma:

Karma account is not for an individual karma. Whenever a person does some thing he/she thinks about many other things. May be some actions are simultaneous. Accounting is done for a group of karma including thinking.
Resultant force decides the effect. This is proved scientifically also. Thinking is the friction force. This may be in the direction or in opposite direction. I will tell a story to elaborate. Once there lived a king. He used to take care of citizens and all citizens loved him. He loved his citizens too. There was a man, a firewood collector in his kingdom. He was honest in his work and used to collect firewood from forest and used to ensure that sufficient firewood is supplied to king's kitchen. He also loved the king. However he always used to think that he must collect sufficient quantity of the best wood with the best smell for the cremation of the king when he dies. He observed that the king didn't like him. He was frustrated why the king should neglect him when he was honest, did his work sincerely and never hesitated to work even after sunset to provide adequate quantity to kitchen. He never failed in his duty. One day he approached a rishi (scholar) to find reason. The rishi thought for some time asked him how he thinks about the king and then told him the reason. Rishi told the firewood collector that he does karma in right way. However, his mind produces friction in the opposite direction. Therefore, the king doesn't like him.

Logic of the story is karma is not only the physical work but also mental thoughts. Effect produced is a combined effect of karma and not independent result for each karma.