I'd bet more libertarian voters just don't show up. My theory is the "Libertarians" that are actually Rs in disguise always fill out the ballot for the R when push comes to shove. Then there are true believers. They hate both major parties and won't show up in a runoff. Finally, there are the disillusioned Rs. If you poke around /r/neoliberal you'll find a huge pipeline of people, myself included, who grew up in conservative areas, became disillusioned with Republicans but still were convinced Democrats are evil, made a stop off at libertarianism, realized democrats aren't evil, and became moderate Democrats.