"What's with all the Impalas with dog dish covers? Didn't the Impalas come with wheel covers?"
Well.....the Impala's were standard with the dog dish hub caps but the cars that I think you are referring too (the '62 red car and the '64 black car) were Super Sports and SS's did not come with dog dish caps.
I don't know what class each of those cars wound up in, perhaps they were not in a Stock Class. If they were not in a stock class then they could have had what ever hub cap the owner wanted to put on them. In the case of the red '62 409 car, I beleive he was in Restored Stock, perhaps that was the only change which would not have been enough to put him into an Altered Class, if that was the case, they would have left him in the Stock Class and points would have been deducted for incorrect caps.