Difference in Karma's power?
Kamma is a peculiar thing. Only a fully enlightened Buddha can understand the workings of Kamma. Also the workings of Kamma is one of the Four Imponderables (acinteyyas).
We have lived countless of lives and all beings therefore have different kamma. Two beings can perform the same act, motivated by the same root, e.g. Greed or Anger, but receive a very different result.
That is based on past kamma. Some beings might have practiced a lot of meditation and done a lot of wholesome deeds in the past, while others may not have done that. That will issue different kammic results to the same volitional action.
Throughout the Sutta Pitaka, there are many examples of the Buddha giving a special meditation course or technique to a being, while another being gets a totally different course or technique.
That is because the Buddha knew the past kamma of a being and thereby which technique would be suitable for exactly that being.
There is an example from the Dhammapada verse 25; The Story of Cūlapanthaka.
Cūlapanthaka was not able to progress in meditation and was very sad about this. He was about to give up his life as a monk and become a lay person again. Then he met the Buddha who comforted him and gave him a piece of white cloth. The Buddha explained to him, that he had to sit in the sun and rub the cloth with his hands, while saying "taking off impurity". He rubbed the cloth which became dirty and Cūlapanthaka realized the impermanence of conditioned phenomena and attained arahantship.
The Buddha gave him this technique because he knew that in the past Cūlapanthaka was a king moving around in the sun and often wiped his head with a piece of cloth and in this former existence of his, he also thought about the impermanence of conditioned phenomena, while seeing the cloth getting dirty.
This time it lead to him becomming and arahant, gaining mastery of analytical knowledge and gaining knowledge of all three Pitakas.