In the DNA sequence there are different variables that build each memory parameter for billions of individuals. The memory prodigy exists, capable of proceeding with multiple examples of performance, some that others find atypical even with training. Who memorizes fast and accurately, and with expectancies of lasting decades!
It must be a variable that controls the rate of oblivion, like a hard drive don't lose data until a certain threshold of age. Then, losing information in the brain has an organic explanation that perhaps it could be modified with an advanced genetic technology.
For Nature is very easy; it modifies this parameter to ensure that the information doesn't exit once is there, imprinted in the cell network. Although to remember information without loses during a lifetime would be problematic for some.
Between different brains, you could expect multiple differences in all the available representations of memory like a set of parameters. A transcendental part of the idiosyncrasy, as it interferes with the behavior in a high ratio. Between two babies, there aren't too many differences but they exist in people with 50 years. Sometimes a very noticeable one, all due to a mixture between memory and the inherent tendencies of the individual, based on a form of training.
The hippocampus was associated with memory processes after different studies. If a brain has an injury in a specific area, it makes easy the process of associating the problem with a difference in results. However the memory goes for more areas, as it was demonstrated.
An incredibly complex topic to study, with multiple opinions about memory parameters and a limited technical understanding after centuries of attention. It's all in the DNA, but a human, assuming a normal speaking rate, would need near a half millennium to simply enunciate nitrogenous bases in the human genome or from other species. To know the topic well looks rather impossible.