Every kit has a bit of play in the steering. It is normal. Maybe some kits have a little more than others, but they all have some. I don't know if the pros have less play in there steering compared to us mortals though.
I don't know why the slack is there, but you're theory sounds about right. Also it only takes a little bit of play between the servo, steering assembly and toe links to create a fairly big amount of play in the front wheels.