I'm just guessing here, as while I am familiar with the health care stuff, I'm not all that "up to snuff" on it. But it sounds to me like you may have had insurance from multiple providers during the tax year, or you have a dependent or spouse that had different insurance or no insurance. But that's just a guess.
Barring any other responses in this thread from someone more knowledgeable on this, I would suggest you call the number provided to you in the letter you received, and ask for clarification. It's also possible the IRS made a mistake, as most likely the letter you got was computer generated and never seen or touched by a human at the IRS, and you are the first set of human eyes to actually see it.