One principle of the karmic law is that it is not automatic. This means that a deed done by a person does not bring a direct effect. In other words, the effect is a mixture of the deed, the conditions in which the deed was done, and the object of the deed.

Let's imagine three kinds of mirrors: a concave mirror, a convex mirror, and a plane mirror. The concave mirror makes you look smaller. The convex mirror makes you look bigger. And the plane mirror can reflect an object twice its size. But in this case, let's suppose that it can only reflect an object of the same size.
The three types of mirrors, concave, convex, and plane, all represent the relationship between the subject and the object of an action. With additional conditions like distance and intensity of light, the rebound of karma is reflected on the mirrors. If a person stands in front of these mirrors, there will be three types of reflections: bigger, smaller and the same size. Simply standing at the same distance from different mirrors brings about different results from the same action.

Let me give you another example. Say that you hit a person who was full of hatred. Then you hit a person who was cloaked with the obscuring energy (Tamas). Thirdly, you hit a practitioner who was wishing to have his karma washed off. You will find three different rebounds of karma from each of these three persons.

In the first case, the person who had strong hatred, you will receive not only the physical karma (the effect of hitting), but also the intense emotional karma, the hatred from the person. In the second case, you will receive not only the karma of hitting someone, but the karma of his ignorance. In the last case, if the practitioner had thought, "Thank you for washing off my karma." when he was hit, you would accumulate the karma of advancing your practice by being hit. As you can see, there are three different effects in three different situations.

This principle also applies to offering, which is a virtuous deed. If you make an offering to an extremely generous person who practices the Four Immeasurables, you are sure to receive its effect in an extremely generous way. If the person is a good businessman, you will also become a good businessman. If the person is highly intellectual, you will pick the fruit of its high intellect. If the person is trying to attain enlightenment, you will also come to direct yourself toward enlightenment. In each case, however, the deed done is the same deed of offering.

From Mahayana Sutra Vol.2