Why can two people who drink the same amount of alcohol have different levels of impairment?

Two people who drink the same amount can have different impairment levels because of individual factors — body weight, biological sex, whether and what they have eaten, hydration, fatigue, medications, and even differences in how their bodies process alcohol. A larger person who has eaten will generally reach a lower BAC than a smaller person drinking on an empty stomach. This variability is why servers should base intoxication judgments on observed signs in each individual rather than assuming a fixed number of drinks affects everyone the same way.