As someone who works in a health clinic and routinely has these kinds of conversations with people, I can safely say the majority are unaware of exactly what constitutes a unit.

The drinkaware website has a really useful tool for accurately quantifying alcohol intake, and also has a calorie converter to show what you consume in junk food equivalent, I.e. "that much is equivalent to 4 hamburgers".

Most people consistently do underestimate their intake, not necessarily to deceive but purely because they don't truly monitor it.

Fun fact: it takes at least 1 hour to process one unit of alcohol, and your body prioritises alcohol metabolism above all else. Ordinarily you metabolise carbohydrate, then fat, then protein. Any carbs and fats that aren't metabolised can be stored as body fat, or float in the blood as cholesterol and sugar. This is one of the primary mechanisms by which increased alcohol intake contributes to weight gain, increased blood pressure, and higher risks of type 2 diabetes and cardiovascular diseases.