Map Dependency Fields Help

Hi,
I have created dependency fields to minimize data entry errors by our reps but this has created an error of its own.

For all of the parent field options that have more than one child field option that applies I also include none so it doesn't default to anything. But by doing this when a rep selects the parent field the child field switches from -none- to none making it a valid option when it really isn't.

Can you please explain why this happens because 'none' isn't a true option within the pick-list but when creating the dependencies it becomes a valid option?
Also can you please help me make it so that once the parent field is selected that none doesn't become valid but it doesn't default to any particular option?