It's often said in UK politics that the Conservatives usually require a 7 percent lead in the popular vote to gain an overall majority of seats in the House of Commons; but the Labour party can get a majority with practically no lead at all (indeed it's not inconceivable that the Conservatives could have a small plurality of votes and Labour still have a majority of seats).

How is this possible?

I know that people tend to spread out from the centres of the towns and cities, but surely that would effect Labour more than it does the Conservatives, who seem to have strong rural constituencies.

Additionally, is this a UK phenomena or does is effect Westminster systems everywhere?