You have a few alternatives to how you can respond to the question, " Why did you/are you leaving your current employer/position"
Keep your response in the "safe mode", short and sweet. The "less is best" theory works the best in the application process. Most job interviewers prefer to save any indepth questions for a live interview
I am leaving,have left, my current/past job because:
1. I am seeking a position in which I can better utilize my skills
2. I am seeking a position with a company(your company) that will enable me to further my career.
3. I am seeking a position that will not only be beneficial to myself but to the company ( your company) I would be working with.
There is no reason you have to, or should, state your personal differences with your current employer, as your reason for leaving your job, to any new potential employer.
Depending on the job you are seeking use your current skill level to promote yourself within that company using "them" as your reason for seeking new employment.
If there is anything further you need, feel free to ask.