(TheStreet) - You can’t believe what is on a wine label. The North Coast California Cabernet Sauvignon proclaims it is 13.9% alcohol by volume - but in actual fact, and permitted in the United States, the real alcohol level can be 12.4% or 15.4%. And that’s a world of difference. 

The Alcohol and Tobacco Tax and Trade Bureau - a cul de sac in the U.S. Treasury - is in charge of wine labels. Here is what its regulation says: “A tolerance of 1%, in the case of wines containing more than 14% of alcohol by volume, and of 1.5%, in the case of wines containing 14% or less of alcohol by volume, will be permitted either above or below the stated percentage.” 

Why this matters: if you are counting alcohol units to stay within healthy guidelines - in the United Kingdom, that’s 14 alcohol units per week, for instance - it is mandatory to know the alcohol content of wines and beers. You can drink a lot more 9% Riesling than you can 15% Zinfandel. A standard pour of an 11% alcohol wine is 1.9 units. The same pour of 14% wine is 2.4 units. But a wine labeled 12.5% might be 11% or 14%. 

You may even cross into DUI territory - just by using wrong, bad alcohol content numbers found on wine labels. 

But how will you know how many units you’ve consumed if you do not know the alcohol count - and you don’t if you are drinking wines bottled in the U.S. and Australia (which also allows a 1.5% variance). 

More disturbing news: a study reported on by the Washington Post found that 60% of wines tested under-reported alcohol content. Only 20% over-reported. 

Also know: the other big fact, now trending, is that alcohol content of wines is steadily increasing in the United States, said Jörn Kleinhans, principal in The SommelierCompany in California. “The trend is for more alcohol in wines," he said. "It is not easy to find reds under 14%.” 

The guess is that we just like a boozier wine - but nobody knows why alcohol content is up, just that it is. 

So why are the labels so misleading? Multiple reasons, said experts. “It’s not intentional," said Keith Wallace, president of the Wine School of Philadelphia. "It’s bad science.” Besides, he added, “you only need estimates to make a good wine.” The winemaker, in other words, has no incentive to get precise in measuring alcohol content. 

Partly, too, it’s just that Mother Nature is in control, according to Sam McCartney, president of SCM Beverage Brokers in Florida. "When the juice is fermented, they take a reading to be sure it meets the minimum standard," he said. "It is not an exact science. Mother Nature takes over and metabolizes additional sugar that increases the alcohol content or doesn't. That’s why the label states an average percent content.”