Your Blood Alcohol Concentration may play a very big role in a DUI arrest. If you're over the legal limit of 0.08% BAC, the law assumes that the alcohol impaired your ability to drive.
However, drinking does not impact everyone in the same way. You have to take all sorts of factors into account, such as how much food you ate or how often you drink. You could have the same amount of drinks as a friend and your BAC could exceed the limit, even when theirs does not. Don't ever assume it's identical.