In 2008, Rowlett was 1st in Area C prelims and yet finished 4th in finals (which unfortunately meant that they were the 1st Alternate). So yes, rankings can and do change from prelims to finals. Sometimes dramatically.
But if your "sometimes bands don't give it their all" in prelims theory is valid (maybe, maybe not), then wouldn't that have to apply to Rowlett as well? Regardless, all 4 of those Area C bands are truly excellent.
One other thought... Area C had exactly 20 5A bands competing this year. Had their been one less, Coppell would not have advanced. Weird.