We have a FPTP system similar to an electoral college where you vote for the MP you want and the party that gets the most MPs wins. So you can have a party with more votes but spread across the wrong areas. Its a sham of democracy that sees the majority of votes wasted.
It basically creates a system where its better to hit lots of little places than go for a large district, very similar to the US EC. In fairness, this did benefit Labour at one point, but recent changes have really made it easier for the Tories.
It's not massively, its just we have a multi party system, if we had a Duopoly it would work very similarly, it's still not based on popularity but number of MPs.