New figures from the Electoral Commission show one in three teenagers of voting age aren’t actually registered to vote, with big holes in the register for everyone under about 40 years old too.

The difference could have a significant effect on the election: younger people are overwhelmingly opposed to the Conservatives and to Brexit, but aren’t as likely to be registered and ultimately turn out as their parents and grandparents.

In fact, it’s this lack of turnout that drives the Tories’ lead in the polls: one reason 2017 was such a surprise result was because turnout was higher than pollsters had expected among under-40s. The weighting was wrongly calculated and experts assumed many people who said they would vote Labour would ultimately not do so. (The size of this effect is disputed.)

But why is there such a gap in registrations between age groups? One reason is obvious if you think about it: older people are older, and have had more time to register to vote.

It isn’t quite that simple though: registering once isn’t necessarily enough to be able to vote, if you’ve moved address. And moving is what younger people do a lot: they are more likely to live in rented accommodation, with all the lack of security that brings, and so have to move house more often.

An owner-occupying pensioner who has lived in the same home for a decade and isn’t moved on by their landlord every few years only has to register to vote once. But with most young people locked out of the housing market and moving constantly, they have to remember to register every time. As a result people forget and fall off the register.

That’s why stories of a “youthquake” that appear before elections can be misleading. Government figures tend to show a big spike in voter registration before elections, and the vast majority of people registering to vote are young people. But this is because those are the people that need to register more often. The system is effectively stacked against them.