The field slopes downward from the center toward each sideline. Although, I don't think it is nearly as noticeable as it was 20 years ago, if it stills exists at all anymore like it maybe did when Hasselbeck played.
I played a few high school games on the field in the early 90's and the slope was very noticeable. I was on the field after the Dolphins game this season and I don't remember noticing a slope at all.