CPR instructors don't deviate. They're not allowed. If they do, the CPR police come in the night and take away their manikins.
Enterprising rescuers -- professional or not -- have come up with tricks of the trade that CPR instructors routinely leave out of the garden variety CPR class. These are the fruits of experience.
"You! Call 911!"
Yeah, and then what? Sending a bystander to call 911 will surely get you a call to 911, but will it get you an ambulance? That depends on how smart your bystander is. When the tow truck arrives, maybe you needed to be a bit more specific about why you wanted him to call.
I always knew breathing was overrated. There's some good science behind the change, so much so that even the CPR police have made this change. They even have a name for it: "Hands-Only CPR."