Actually, were I live I believe there is a legal difference between DWI and DUI.
I can't remember which is which, but one is when a police officer determines that you are intoxicated regardless of physical evidence. For example, you can pass a breathalyzer and still be charged if the officer says you are impaired in his opinion. The other is when the Blood Alcohol Level is above .08%. That is an automatic charge.
So, in actuality it is a two-pronged charge. You can pass a breathalyzer and be charged with one charge. However, if you fail the breathalyzer, you are charged with two offences...