What happened for some respondents whose responses are in progress?

There are a few respondents whose responses are in progress in the "Data & Analysis" tab. For some respondents, their "last question" column are blank. When I click these cases, I found even the first question are "not displayed to the respondent". So I want to know what happened for these a few respondents? Did they just click the survey link in the email, but then close the survey soon? But it doesn't make sense, since even in this way, they should leave a record of displaying 1st question.