Alan,
What can also happen is that the fish hits the dry, but misses it, gettting the LINE from the dropper in it's mouth in the process. The dropper fly hooks the fish as the line is pulled through the trouts jaws.
Or, the fish hits the dry, you hook it, but it comes loose during the fight, with the same result as above.
I have also seen the fish have BOTH flies in it's mouth, but my speculation has always been that the fish hit the dropper fly first.... Could go either way, I guess.
Good luck!
Buddy
It Just Doesn't Matter....