They probably mean "units" rather than drinks, and quite a lot of standard measures - glass of wine, pint of beer - are two units. It implies habit, such as having a drink when you come home from work and then maybe another one later, plus partying at the weekend, which easily adds up to those averages.
Quite a lot of people drink a lot in a way that's mostly invisible.
Quite a lot of people drink a lot in a way that's mostly invisible.