Hola Ashlyn

The voices in our dictations are all professional actors but in a dictation the narrator needs to be very careful with the pronunciation, so that the students hear very clearly what is being said and they can transcribe what they hear. This is the main purpose of dictations. 

On TV people talk very naturally without any pedagogical purpose and this is why you will always find this difference.

I hope this clarifies it.

Saludos