This horse's responses may also be getting better because she has only one person riding her and therefore only one set of aids to understand.
I can't think of anything more boring/depressing for a horse than being a lesson horse for several riders. We bought the horse my grandson was taking lessons on and later, when Gabe outgrew hum, sold him to a teen beginner rider. She uses him in high school equestrian team and takes the wonderful care of him that many girls lavish on their horses. It's been a very nice move for Dakota. Nancy