Think about mixing drinks: You can only make a certain number of gin and tonics with one bottle of gin.

FORBES: Running in place

They suggest it may be down to drinking patterns and habits - those that are mixing drinks, and drinking at different venues, may be more likely to under-report.

BBC: Level of excess drinking of alcohol 'is underestimated'

Many now use elegant long spoons made only in Japan for mixing drinks, and several have even adopted the Japanese practice of carving perfectly round ice balls.

WSJ: The Ascent of Japanese Whisky | The Cult of | WSJ. Magazine May 2013