statement of purpose
Your CV should explain well what you have done in the past. Your SOP should be more future directed while also attempting to bridge the past and future. But don't think you just need to explain what is in your CV and neglect what you see as your future. Think about what you want to do and what you have already done that puts you on the path to achieving your future plans. So, yes, say something about your quantitative abilities and such. 

People will understand that others may want to change fields for a variety of reasons. You aren't unique in that so don't dwell too much on it. Say where you want to go, what you've done to get there already, and how this program will help you even more. But put the focus on the future, and not just the program you are applying for, but beyond that.