Well, for one thing, we no longer have the same Prime Minister -- or, at least, won't do come October. For another I'm not even sure we'll have the same Labour Party in about three months either. And then hasn't UKIP's stock risen? They were shut out of the election a year back and their central policy aim has struck back with a vengeance and then some. But no-one at the moment seems willing to do anything about it apart from destroy each other. Maybe their stock has risen some since being granted only one MP on the back of 2-3 million votes?
Up in Scotland, I don't think it's just the SNP that's grumbling about wanting to leave the UK in and attempt to stay inside the EU...
And finally, while we have voted to leave the EU, that means all sorts of different things. Out of literally everything? Only part way out? Some other subtleties? It could be UKIP's version of out vs. a "soft out", or some such. The public might want to have their say on that.
Materially, loads has changed in the UK parliament in just the last three days, let alone the last year. An early General Election may not be inevitable but it's not totally unreasonable in these circumstances.