Maybe, Charlotte "uses" riding for having sponsors and signing contracts with companies like Gucci, Cartier etc who support her.Zara is from a family who take equestrian pursuits very, very seriously and she's the daughter of two Olympic riders. She was probably being instructed on proper form from the time she was a three year old on her first pony ride. She's also from a family that doesn't go in much for flash and glamour.
Charlotte on the other hand- no one else in the family is into equestrian pursuits and I suspect she sees riding as part of a jet set, glamorous lifestyle. She's said as much in interviews- that it's the ultimate glamour sport.
I imagine Zara, coated in mud from doing a cross country course or driving the horse van or sitting up with a sick horse would howl in laughter at the idea of riding as a glamour sport.
Charlotte sees riding as a part time thing she can do on her own schedule- just show up well groomed and lovely, jump on a horse someone else trained and groomed and ride the course- then hand the reins back over to a groom. She doesn't form a partnership with any of her horses and they don't trust her or like how she rides and it shows.