I had a feeling. In my second playthrough I was able to get her affection over 50 points with no dominant points but in doing so I got over 80 affection points and around 20 submissive points for Eve. I also received over 30 affection points for Kelly as well. I'm guessing the moment you choose between Eve & Cassie pretty early on dictates who takes to you faster?
Kelly will always be late to the party but you should be able to grow affection for both Eve and Cassie at a similar pace. If you do engage "romantically" with one or another, you will get significantly more affinity points. That being said, the affinity score is important, but what really matters are the choices you make.