While some have commented that Morgan Freeman's voice changes dramatically throughout the film, it is a major plot point that he is living a lie, living under an assumed identity and not particularly good at it.
It is related to the theme of doing good, even though you may not be particularly good at it, nor be able to continue doing it for very long. It sounds unnatural for Morgan because it is unnatural for his character.
During a scene at the soup kitchen, Miles wants help serving the soup and shoots a slice of bread at Manual to get his attention. The bread lands on Manual's tray next to his soup bowl. In the next shot, the slice of bread has disappeared.