My reasoning being, that I believe when polled, many people are much happier to say 'don't know', rather than declare their preference in case they are viewed as being supporting a seemingly unpopular choice. After all, what kind of person would have voted for smarmy Dave again, apart from those pesky purple rinsed conservatives.
Similarly, I believe that many 'don't knows' in the EU poll, are those who want to avoid being seen as anti immigrant etc. After all, aren't we all broad minded liberals at heart
Come election day and in the privacy of the polling booth, I expect many of these people to vote 'leave'. You can currently get around 3/1 on leave. A good bet