The General Election on this day in 1951 couldn’t have been more decisive. Six years since Labour’s victory in 1945, and one year after their victory in 1950, the public was still behind Clement Atlee reforming agenda. The chances looked good for Labour in 1951 and, as the results started coming in, the numbers looked good as well.

They had won the most votes any political party in any election, a record not surpassed until the Conservative Party’s victory in 1992 – when there were over seven million more people in the country.

In fact, even with a smaller population, Labour won the most votes they have ever won to date.

On top of their record-breaking haul of votes, Labour was well out in front, with almost a quarter of a million votes more than the Conservatives. A decisive victory for Labour then? Not quite.

Clement Atlee had to resign as Prime Minister and an ageing Winston Churchill quickly moved back into Downing Street, while his Conservatives formed a government with a majority of 17.

The 1951 General Election was certainly quick and decisive – and one party with a majority was kicked out of office to be replaced by another. It’s just that the wrong party got kicked out.

This wasn’t the last time a ‘wrong winner’ vote – where a party gets more votes but fewer seats than its opponent – would happen. In February 1974, Labour secured 301 seats to 297 for the Conservatives – despite the Conservatives beating Labour by over 200,000 votes.

Internationally, there are other precedents. New Zealand saw two wrong winner elections in a row in 1978 and 1981, setting them on the path to electoral reform. The mechanics of the electoral college in the United States are also similar and have delivered Presidents who did not win the popular vote in 1876, 1888, 2000 and 2016. And provinces across Canada are now debating adopting a fair, proportional voting system after a series of undemocratic wrong winner elections.

Why? The results of elections under Westminster’s First Past the Post system aren’t in proportion to the votes that are cast.

Due to the complex way votes are turned into seats, some parties will get significantly more seats than you’d expect, while others get significantly less. When two parties get a similar number of votes, these differences can turn a loser into a winner and vice-versa.

Put simply, if a party wins huge majorities in constituencies – such as the 77% majority for Labour in Liverpool Walton in 2017 – then those additional votes above and beyond the nearest challengers do not help a party win any more seats.

So, the most ‘efficient’ thing to do under Westminster’s voting system is to try and win constituencies by a small margin – not to ‘waste’ effort in seats you are likely to lose in or get a bigger majority than you need.

The result is a huge amount of votes not counting towards getting anyone elected to Parliament. And of course, a skewed campaign map, with swathes of the country written off by parties as ‘unwinnable’ or ‘safe’.

In 1951, Labour was pilling on votes in seats they had already won, while the Conservatives won narrow victories. The opposite happened in 1974 when the system meant the Conservatives lost out to Labour. Since 2015, the problem of electoral ‘bias’ means Westminster’s voting system has advantaged the Conservatives.

Any system that has constituencies will have some bias in it, but the more proportional a system the less bias – thereby making sure that the parliament the public vote for is the one they get.

In a simple electoral system, you wouldn’t have to calculate complicated uniform swings to find out how many MPs a party with half the vote would get. They would get half the MPs.

When voters know what impact their vote will have, they are powerful. When only party analysts can work out the impact of electoral results on parliament, they are the ones who are in control.

If a voting system can’t even pass the first test of democracy – ensuring the winner actually has more support than the runner up – it is unfit for purpose. We need to fix this broken system before it happens again.