You ask an intriguing question.
Historically, beers were a food source in addition to being
a recreational drink.
Could the additional calories be from unfermented sugars,
or grain protein or starches in some form?
While in the Navy I drank many a beer throughout the
asian countries/islands and never found one I didn't like.
Some had very unique flavors and varying degrees of
bitterness and coloration, but were delicious. Generally
much better than most American brewed beers.