Cameron had form for manipulative referendums.  When the coalition with the Lib Dems was formed, the lib dems wanted to change the electoral system to Proportional Representation.  The tories agreed to a referendum giving a choice of not PR but some horrendously complicated system or staying as we were.  As hoped, people voted to stay as we were, enabling Cameron to tell the lib dems to shut up about it.

A few years later, he attempted the same thing to shut up the anti-EU wing of the tory party.  He was betting that, as previously, the masses would vote for the comfortable "stay as we are" option rather than the "take a risk on change" one.

It all proves how a gamble is always a gamble, even if it's apparently a safe one.