It's a guess. One based on a simple assumption that protestors are politically active, and don't sit on their asses as much as the average population member does. Therefore, it stands to reason that they wouldn't sit on their asses during voting times either.
It could be completely wrong. Even if it's not, flavio's argument is irrelevant. He's arguing over what the percentage actually is, when the bottom line is that it doesn't matter what the percentage actually is. The original post, and the argument contained therin, is valid regardless.