David Cameron also tried to negotiate with the EU and failed. The, after barely winning the elections, he called for brexit referendum, thinking UK voters would

David Cameron also tried to negotiate with the EU and failed. The, after barely winning the elections, he called for brexit referendum, thinking UK voters would vote to stay with the votes being split at around 50/50. He would then have used this result for a new round of negotiations. But things worked out quite a bit differently. How is this any different than what Varoufakis tried to pull off?