Karma of Birth?

I've heard even though two people do the same good Karma their returns can be different on what they were thinking.

I am not sure if you meant:

1. the same action
2. action giving same level nature of fabrication
3. actions giving the same results

With regards to 1, if two people to the same action. E.g. Cutting open someone's stomach. Then a surgeon maybe motivated to cure the patient while a robber to kill. This volition decides the result through the actions are the same.

With regard to 2, though you may gain Karma which are bound to give the same results there a many other interaction from other Karma which hinder the effects of certain Karma. In addition, Niyama Dhamma also may play some role on the effects of Karma. If 2 people have the same set of fabrication with the same potent, how they came to being through volition and who they give results will be different.

With regard to 3, there may be times that 2 people may experience the same circumstances. Though the circumstances are the same, the initial actions, interfering Karma and Niyama Dhammas interacting would mean the action and volitions leading to this will be different through the final circumstances are affectively the same.

And when coming to effect these different Karmas can give different births to the beings who did those Karma Unlocking or Locking the ability to realize Nirvana in that life itself.

Though Nirvana is un fabricated the path to Nirvana is fabricated. Karma cannot make you realise nirvana but will give the supporting conditions to help your towards it like coming in contact with the Dhamma, proper places to practice, etc. Again the diversity of volition, diversity of other fabrication, diversity or Niyamas in effect, randomness all play a role in the final outcome or aligning of all condition to even support your path towards Nirvana.

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.