… the deed in the early text [MN135] is simply stated to be the killing, or refraining from killing, of living beings, and so on. The specific types of actions, and their approval are not mentioned. In the [later] Sanskrit text we get a list of normally around ten causes that lead to the result, many of which are illustrated
In the centuries after the Buddha, many of the subtleties of karma were simplified for didactic expedience. This led to a formulaic, “if you do this, this will happen to you” understanding of karma (which the Buddha himself rejected as fatalistic). This model came to be repeated ad-infinitum in texts (such as the Karma-Vibanga) and in Buddhist art (such as at Borobudur) for millennia, perpetuating a simplistic, “popular” understanding of Karma which persists today.