How can a person who has practiced virtue their entire life be reborn in the lower realms? Well, even though the person may have been virtuous throughout this life, we do not know anything about this person’s previous lives. Maybe the person has been virtuous in this as well as the last two lifetimes, but such might not be the case if we could go back even further. Some negative karma might have been committed many lifetimes ago. From the perspective of the three types of immutable karma, virtuous karma that the person has committed in this life might not happen to ripen in the current or the next life, but in some indeterminate future lives. That is, it may not come to fruition until perhaps hundreds or even thousands of years have gone by.
~ Luminous Wisdom 1: On Cause and Effect