The confusion might come from thinking that libertarians are a homogenous group. To make a generalization: some groups of libertarians do value liberty as an end in itself, or the ultimate moral value; others see it as the logical extension of acting on a utilitarian calculus. Given my previous involved in the libertarian community and what I know about multi-system moral psychology, this seems plausible, if not obvious. [PDF warning]

But to an outsider researcher not already familiar with the libertarian movement/mindset, I can see how they wouldn't know that and how that lack of knowledge could skew the data into the contradiction you identified.