DNA evidence analysis is an imperfect science. Provide two reasons why this may be the case. For my Biomedical Science Class

Not all DNA that was found at the scene has to be related to the scene.

Explanation:

Just because DNA was found at the scene, that doesn't mean that the person there is actually guilty. For example, in a case where person B is guilty, a strand of hair from Person A could be carried over to another scene on Persons B's clothing.