Barbarino,
You may know a lot about credit but you clearly don't know as much as you think about the auto industry. I work in it and know it very well (no boo's please we all have to make a living). 0% is offered by some manufactures but you cannot get that kind of rate just with a good credit score. The way it really works actually has more to do than just score. The automobile financing companies tier everyone out. Your score is the biggest part of the equation, however past automobile history in particular changes things as well as percentage financed relative to invoice. That being said different companies accept different tiers to qualify. My company uses tiers o (the elite), 1 and 2 and all people in those tiers qualify for the same rate. Honda for example usually only qualifies their top tier! Most domestic manufactures tier easier and buy deeper with their special rates. Manufacturer's don't own the vehicle you see on the lot, so they care less than you think if these cars are sold. The only reason they care is when a dealership sells a car, they need a new one to replace hence why they offer support (incentives).
Simply put if the manufacturer isn't offering 0% to keep the assembly line moving then you will NOT get 0 % even if you have an 850 credit score. If you received 0% with a 640 credit score you probably did it with GMAC (they buy deepest of all) or everything lined up very well for you the way you tiered out.