This is America, But We Don't All Speak the Same English
Clearly, if you were raised in Jackson, Mississippi, you are going to sound very different from someone who was raised in Boston, Massachusetts. However, American speaking styles are much more nuanced than just a deep south to northeast comparison.
For example, my home state of New York is home to approximately 3 different styles of speaking: eastern north, north, and greater New York City. Another, Justin’s home of Nebraska, is also a state with multiple accents; ranging from the midland, west midland and west styles.
So why the differences? Well for one, accents depend upon the position of your tongue and how you open your mouth when saying specific words.
When my mother (who has a very thick New York accent) says the word ‘water,’ her mouth will have a smaller opening in the first syllable and will only get slightly larger in the second syllable. Justin, who has more of a west accent, will change shape and openness of his mouth such that he goes from a wider opening in the first syllable to a smaller opening in the second syllable. The result is, my mother's prononciation will sound like 'wouh-duh,' while Justin's prononciation will sound more like 'wah-terr.'