Hmm... Okay, fine, I'll take a crack at it.
Approaching this as a proofreader (I have done a fair amount of professional proofreading/editing), and not as a GMAT expert, I'd say the sentence is fine. Because "in the United States" is a prepositional phrase, it's completely clear that it's set up to be compared to "in both Spain and Ireland," so we know what the sentence means. I think if the GMAT were to give us this error, they'd give us "...more than twice as much as the United States" so that the error is more stark and therefore less debatable. Remember, the problem with debatable questions is that there won't be consistent answers among test-takers, and the question will end up being thrown out by the CAT algorithm.
One way to clarify it, from a GMAT perspective, is simply to add a pronoun to relate back to the construction: "...more than twice as much as
it does in the United States." Here, "it does" would substitute for the proper comparison, "construction accounts" (subject and verb). Even this, though, is awkward; as people have pointed out above, the placement of "more than twice as much..." isn't ideal.
Usually, in this situation, the fix the GMAT will give will be to add a noun and its associated modifiers. Therefore, I think the most likely GMAT fix would be:
In 2007, construction accounted for 13 percent of total employment in both Spain and Ireland,
a figure more than twice as high as that in the United States.
Now it's clear that we're comparing two numbers, the percentages, and there is no more confusion.
Once again, this isn't really a good GMAT discussion, because we're not given answer choices to compare, because I'm not sure the original sentence is objectively wrong, and because we should never predict the right answer because such prediction is rarely accurate. Nevertheless, I freely admit it was fun to attempt to fix, so, thanks
