An identical drink produces different blood alcohol concentrations in different people, sometimes by a wide margin. The variables are physical and enzymatic rather than a matter of habit or willpower.
Alcohol distributes into body water
Ethanol is water-soluble and spreads through the body's water compartment rather than through its total mass.
A person with more body water dilutes the same quantity of alcohol into a larger volume, producing a lower concentration.
Because muscle holds far more water than fat does, two people of identical weight can reach noticeably different concentrations from the same drink.
Enzyme activity varies between individuals
Two enzymes do most of the work, one converting ethanol to acetaldehyde and a second converting acetaldehyde onward to a harmless product.
Genetic variants of both are common, and some variants work considerably faster or slower than others.
A variant that allows acetaldehyde to accumulate produces flushing and discomfort quickly, which is a metabolic difference rather than a low tolerance in any behavioural sense.
Stomach contents change the rate of arrival
Alcohol is absorbed mainly in the small intestine, so anything that slows the passage from the stomach slows absorption.
Food, particularly fat and protein, closes the valve at the base of the stomach for longer and spreads the same dose over a longer period.
The peak concentration reached can differ substantially between an empty and a full stomach even though the total alcohol is identical.
Drinking speed matters more than most people assume
Clearance proceeds at a roughly constant rate that cannot be accelerated, so concentration is set by how far intake outpaces it.
The same total spread over four hours produces a much lower peak than the same total in one.
Carbonation adds to this by speeding stomach emptying, which is part of why sparkling drinks are felt faster than still ones of the same strength.
What this means for counting units
A standard drink measures what was consumed. It says nothing about the concentration that will result in any particular person.
The count remains useful for tracking intake over time, which is what it was designed for, and unreliable for predicting how an individual evening will go.
Sex differences in average body composition and enzyme activity are the reason published guidance is often issued separately for men and women, which reflects population averages rather than any individual's physiology.
Medication adds a further variable, since several common drug classes compete for the same liver pathways and change how quickly alcohol is cleared.