Indeed. Many people (though not all people) lie on anonymous polls. Frequently. They lie when they perceive themselves to have incentives to lie (per your last bullet point). They also lie in absence of any apparent incentives.

Some of it is just human psychology. Nobody sees what appears to be a norm, measures himself against it, and wants to admit that he falls short of it -- even if he's totally anonymous. This is especially true when a poll concerns topics of great emotional weight, like salary. Take an anonymous poll on everyone's favorite sports team, or favorite ice cream flavor, and you'd probably get accurate results. Take a poll on salary, and many of the results would be dubious.