Why is the third person point of view unreliable?
You never know if they are altering the truth or simply not telling the truth at all for one reason or another weather it be there personality, to hide something, or to make themselves look better.
First-person narrators are often unreliable, because they often have an ax to grind, they're children, they're handicapped somehow, or for other reasons. The convention--the normal assumption--is that a third-person narrator is reliable, unless the text contains evidence that the narrator is not reliable. In fact it's hard to think of an unreliable third-person narrator.
Some examples of a point of view are first person point of view, second person point of view, and third person point of view. A first person point of view is told from the view of the writer or narrator. "I went to the market." Second person point of view is told to the person who is reading. "You went to the market." Third person point of view is told about another person as if…
The third person point of view uses words such as they, he, she, etc. and it is impersonal (it does not mention the words I or we). If you imagine a person looking at an event, they are in the third person point of view and they are just seeing what happens. The third person is not 'involved' so that is why it is not personal.