A miami is a normal with a certain variant of coloration. Normal cornsnakes vary widely in the amount of red, yellow, and black pigment per individual. A certain "look" is called "miami" because around the Miami, FLA, area, there have been found individual cornsnakes with a light grey groundcolor, but there are many variations even on the miami theme. Some "miamis" have a brownish ground color, and many darken with age.
Its entirely possible that your snake WAS born of miami parents, but is a bit differently colored than they were. It is also possible that it was born of other parentage, and had a "miami-phase" look when it was hatched.
Its not a bad looking snake, and I'd say it looks like it has miami influences.
Just My Opinion,
-Jack
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...and I think to myself, "What a wonderful world."