Looking at the two sets of legs, I see a couple things:
She probably does full range of movements, emphasizes lunges, contractions and has found her magical rep range. She probably also does heavy movements, but not overly heavy where she's piling on too much weight and the movement suffers because of this.
She could have favorable slow-twitch muscle fiber ratio, so when a trainer told her to do "higher reps to tone" her thighs, this worked because of the muscle fiber ratio. Thus her magical rep scheme is born.
The legs on the right, need to do plenty of movements; deep BB squats, front squats, hack squats, finish off the routine with leg extensions, and do a variety of rep ranges. One week, or two, you do heavy squats for many good sets. Then you up the rep range a bit and do hack squats for a similar number of sets...go back to heavy days, then I'd do an all-out assault and make sure you can't walk out of the gym; dropsets, super/tri-sets and 20rp squats to finish off your brutal workout.
Of course, diet plays a role too.
