I think there are several ways Coppell could theoretically get a 17. (Note: I wouldn't have given Coppell 17. I'm arguing possible scenarios.)
1) Coppell immediately followed Marcus in prelims. Besides Duncanville, I think anyone following Marcus would see their score a little depressed in music.
2) Look at the other prelim scores from the same judge. He obviously valued difficulty. (For example, Haltom at 2. IMHO, Haltom played the hardest book of the day by far.)
3) Coppell loses points from a music ensemble perspective because of individuals who stick out. (Bass Trombone, for one.) Also (IMHO) in general their brass blasted and didn't play with resonance (much like Bowie, who were also scored low by the same judge.)
Here's the show if you weren't there/don't remember:
I don't feel great picking apart the show, I really enjoyed it and had them easily ranked in the top 10. During finals, I pulled up the prelim scores and was shocked by the 17. I spent their finals performance trying to see how someone could come up with that ranking. It's possible. Music judges listen for different things (and that's okay). That's why there are three of them.