Why do the Mannings sound like they have a Different accent ??

In summary, the Manning kids, especially Peyton, are from New Orleans but in terms of culture are not totally part of New Orleans. Indeed, in terms of culture, they probably are a tad more from Mississippi--in speech, in manner, in outlook--than from New Orleans. And the reason is Archie and Olivia are from Mississippi, and the family's roots are there.

That nails it. That and they also spent alot of time in Mississippi during the summer months when they were growing up.....But hey who says talking like you are from MS is a bad thing.