Could this be something where contacting non-responders or sending out additional questionnaires could skew the results? Maybe non-response is considered part of a poll and by fighting against that you would somehow be putting your thumb on the scale?
I'm not an expert either in statistics or in polling; so I'm just speculating.
It certainly could and depends on the questions being asked. If you are sending out a questionnaire via email and some of the questions are along the lines of "how many times a day do you check your email?" or "how much time do you spend at your computer?" you could certainly get some skewed results.
I'm not an expert either in statistics or in polling; so I'm just speculating.