Oscillation ongoing in ORB offering : why are online and telephone pollsters light-years apart on the EU referendum?

As you're probably aware, there was an ORB poll earlier this week suggesting that the EU referendum is pretty tight at this early stage - before rounding, the Yes/"In" side are ahead by just 54.6% to 45.4%.  But it was only last week that we saw an Ipsos-Mori poll putting the Yes campaign almost out of sight, with a 3-1 lead.  There again, it's only been a couple of weeks since we saw a YouGov poll suggesting a very similar state of play to ORB.

Unfortunately, we're just going to have to get used to the same sort of madhouse commentary we saw during the independence referendum, with the media comparing apples with oranges, and breathlessly claiming there have been wild and decisive swings of opinion when in fact nothing much has changed.  Britain has not gone from being evenly divided to being heavily pro-EU - and then back again - all in the space of two rather uneventful weeks.  Instead, the difference in the numbers is caused by the data collection method - Yes are consistently polling much, much more strongly in telephone polls than in online polls.  Terrifyingly, that means we don't have a clue what the true state of play is, other than the fact that No probably don't have an outright lead.

But what is causing the divergence?  It's likely to be one of the following possibilities, or a combination of both -

1) Politically committed people who sign up for volunteer online polling panels are disproportionately anti-EU, and standard weighting techniques are unable to correct for that.

2) People are too embarrassed to tell a telephone interviewer that they want to leave the EU.

Obviously if the first possibility is correct, the big Yes lead in telephone polls is more accurate.  The opposite is true if the second possibility applies.  Take your pick.