I think when fans aren't attached to or invested in any dancers they tend to vote for the most improved. Watching somebody improve is more interesting than somebody who nails it every time.
Bristol is more intriguing than Kelly Osbourne because she had no background in either dance or entertainment. She had to overcome stage fright, lack of athletic experience (and cardio), lack of dance experience, and lack of performing experience, to say nothing of her age.
I think it's a remarkable achievement... especially when she hasn't shown any attitude like some other stars. She's generated a lot of good will for any future public endeavors.