We've gone over this before. A promise made by a government in the form of a leaflet sent to every household in the country is a much more binding promise than those promises made by the campaigning groups on either side of the debate.
We expect campaigners on both sides of an issue for a referendum or election to make wild promises: if we're being generous we say they're exaggerating - that the promises aren't really promises - they're mere aspirations; if we're being harsher we say they're lying. The side that expect to lose typically make wilder promises than the side that expects to win: the losers, after all, will never have to deliver on their promises.
This is very different from the promise made by a government that will still be in power after a referendum. When a government promises, "this is your decision; the government will implement what you decide" then voters rightly expect the government to keep that promise and will punish very harshly any foolish MPs that attempt to renege on it.