Michael Fallon resigned from his position as Defence Secretary because he had “fallen below the high standards that we require of the Armed Forces that I have the honour to represent”.

But apparently, not below the standards of a backbencher in the House of Commons, which will now be Fallon’s job.

Isn’t it a bit weird that when politicians resign from their positions for bad behaviour, they are admitting that what they’ve done is inappropriate (or worse), but not quite enough to mean they can’t hold public office and represent constituents in the mother of parliaments?

Only three by-elections were triggered due to the expenses scandal, for example. There were hardly any resignations from parliament. And the two most scandal-hit MPs recently – Jared O’Mara and Michael Fallon – remain in the Commons, having resigned from the other positions they held.

Being an MP is a position of great responsibility, prestige, and influence. Why doesn’t this matter when it comes to poor behaviour, when being in cabinet, shadow cabinet or on a select committee apparently does?

The reasons are pretty obvious – triggering a by-election can be harmful to the party, your constituents can vote you out (or your party can deselect you) at the next election anyway, and it would set a difficult standard for other dodgy MPs on all sides of the House – so parties would rarely push for their opponents to do it.

But this still means we’ve accepted it as the norm that, say, Tristram Hunt can just leave in the middle of a parliamentary term for a lovely job running the V&A and trigger a by-election, but Fallon – who appears to have at least one misdemenour in his past – gets to carry on being paid to serve the people of Sevenoaks, with all the privilege that comes with that job.

The first is undesirable, or even immoral as my colleague Anna Leszkiewicz argued so well at the time, and the second is the same, for opposite reasons.

MPs are there to serve and lead. They can’t represent their constituents’ interests and set an example if their behaviour has been deemed inappropriate enough to be sacked from another high-profile public office job they happen to hold.

This odd acceptance has been noticed everywhere from the satirical website Daily Mash to The Times’ defence editor.

“As a Sevenoaks constituent, why does Sir Michael Fallon think our standards are suitably lower than military’s that he can still serve us?” asked the latter, Deborah Haynes, on Twitter. “I find it – as a constituent, Fallon’s my MP – presumptuous of him that we are happy to accept whatever transgression he’s done.”

The Daily Mash went for the slightly less measured: “I’m too dirty to be a minister but just dirty enough to be an MP, says Fallon”.

Professor of Politics at Birkbeck, Rosie Campbell, says this trend is “a kind of half apology”, and a way for politicians to “send a nod to the electorate or consumers of media that they’ve done something wrong, that they’re going to apologise by standing down from their ministerial position” but remain as an MP.

“If you don’t step down from your position as an MP then your career can be revived at a later date,” she says. “You can sit on the backbenches, pay your penance, then you can be resuscitated and brought back from the dead” – like the current International Trade Secretary Liam Fox, or the former business secretary Peter Mandelson when he returned to cabinet after resigning twice from government roles.

“To step down from being an MP, there’s no chance of coming back to life,” Professor Campbell points out. “You’re saying ‘I accept this verdict on my character.’ So to wait until the next election when you quietly retire is a much securer way of giving your reputation a chance to recover.”

Numerous MPs were blocked from standing again in 2010 following the expenses scandal, and some decided not to run again. This is usually what happens when an MP has the whip suspended, or hasn’t broken the law to the extent that they have to stand down during a parliamentary term.

You are only removed from your seat if the crime you’re convicted of is “serious” – over a year’s prison sentence. Any MP detained in the UK for more than a year is disqualified from membership of the House of Commons and their seat is vacated.

Last century, only five MPs left or were expelled before or after conviction and imprisonment on criminal charges. So “there’s a lot of defensiveness around the idea that MPs should resign in other circumstances, such as scandal”, says the Institute for Government’s Dr Cath Haddon, an expert in the constitutional issues of Whitehall.

“It’s about a different concept of expectations around the role,” she adds. “It’s almost like a higher standard for ministers that it reflects on the government, whereas for an MP it’s seen as reflecting on them as an individual.

“It’s represented as some kind of reputational thing, and therefore the judgement of electors is believed to be at the next election. This is why the issue of recall and deselection are so hotly contested.”