Republican frontrunner Donald J. Trump has a massive lead in national polls. But it may be even bigger than it seems, because while most major polls are conducted by telephone, his lead in online polls is even bigger.

Telephone polls–especially those restricted to land lines–tend to miss a certain number of voters. Online polls tend to exaggerate the support of well-organized candidates like Ron Paul–who consistently won online polls in 2008 and 2012–and celebrity candidates like Trump. But experts are starting to wonder if Trump’s online polls may suggest a real popularity among people traditional polls are not reaching or who withhold their vote from pollsters.

David Lauter, writing for the Los Angeles Times, notes Monday:

The analysis, by Morning Consult, a polling and market research company, looked at an odd occurrence that has cropped up repeatedly this year: Trump generally has done better in online polls than in surveys done by phone. The firm conducted an experiment aimed at understanding why that happens and which polls are more accurate — online surveys that have tended to show Trump with support of nearly four-in-10 GOP voters or the telephone surveys that have typically shown him with the backing of one-third or fewer. Their results suggest that the higher figure probably provides the more accurate measure. Some significant number of Trump supporters, especially those with college educations, are “less likely to say that they support him when they’re talking to a live human” than when they are in the “anonymous environment” of an online survey, said the firm’s polling director, Kyle Dropp.

Earlier Monday, Trump’s rival, Sen. Rand Paul (R-KY), suggested that polls showing Trump with a huge, 20-point lead may be exaggerating. In fact, the error may swing in exactly the opposite direction.