Lagunitas IPA

I like this beer. Its one of those "in-between" IPA's where I don't get much hops, fruit, or malt presence. But it's well balance and tasty. If I drink 3-4 during the course of an afternoon into the evening, while BBQing, I get a mild alcohol buzz. However, when I drink SN's Torpedo, I have no buzz whatsoever. Even if drinking the same amount, during the BBQ or same time frame. This has happened 2-3 times. Lagunitas IPA is 6.2%. Yet Torpedo is 7.2%. How accurate are the ABV's on beers? Or is there another factor involved? I don't drink to get drunk, but comparing these two brews brings a noticeable difference in alcohol content.