I have been a business litigation attorney for nearly 25 years and have been lead trial counsel in about 60 trials. A few years back I was the lead trial attorney in an extensive lawsuit for unpaid wages filed by 36 former employees of my client. The trial lasted 5 months. The jury came back with a verdict after one month of deliberations with a mixed verdict finding mostly in favor of my client but awarding about 25% of the money sought to some of the plaintiffs.
After this experience I went on a quest to better understand how juries decide cases. Through my research I came up with what I think is the answer. Most people have what I will refer to as a schema. This is a combination of their experiences, values, opinions, upbringing and biases. For example, assuming the case is for medical malpractice. Some prospective jurors going in may believe that doctors are angels that can do no wrong. Others may think that doctors are part of a conspiracy with pharmaceutical companies and cannot be trusted. These later types usually believe in holistic medicine and do not like or believe doctors.
When evidence is presented, the people believing doctors are angels will filter that evidence and remember only the evidence that supports their schema. The people with the opposite schema will only remember or believe evidence that supports their schema. Each side will then come up a version of the facts consistent with the evidence they chose to accept.
It is the trial lawyers job to question prospective jurors during the voir dire process to uncover any such bias or prejudice.