It’s deja vu in Egypt, with huge demonstrations at the weekend against president Mohammed Morsi raising memories of the revolution that overthrew his predecessor, Hosni Mubarak, almost two and a half years ago, at the height of the Arab Spring.

Revolution wasn’t meant to be easy; indeed it’s proverbial that revolutions devour their own children. Morsi may end up being one of those. The array of people massed in the streets against him is truly impressive: clearly his year in power has been unsatisfactory for many, with economic problems, political exclusivity and fundamentalist influence on social issues being among the grievances mentioned.

But unseating a democratically elected president is a different thing from deposing a dictator. So different in fact that it’s hard to think of a single case where it’s been achieved by popular protest.

It’s not hard to understand why that would be the case – why risk life and limb in street violence to do something that you can do peacefully at the next election? And if the numbers are not going to be there for an election victory, it’s unlikely they’ll be there for insurrection.

Egyptians who stood up against Mubarak knew they were taking a terrible risk. Without disparaging the motives or the determination of the anti-Morsi demonstrators, it’s fair to say that the risk they are taking – at least at the moment – is much less. We need to see how well their numbers hold up before assuming that they can repeat the success of 2011.

The other thing to reflect on is that election systems matter. After the fall of Mubarak, Egypt chose a new president by a standard two-round election, as used in many impeccably democratic countries (France is probably the most familiar example).

A first round was held with a large number of candidates, and the two leading ones (assuming neither of them won an absolute majority, which in the circumstances was a safe assumption) contested a runoff the following month. Morsi won that narrowly but clearly, with 51.7% against 48.3% for Ahmed Shafiq. (Adam Carr has the figures.)

The problem was in the first round. The two-round system works best in situations where everyone knows who the final two candidates will be, and the purpose of the first round is mainly to let a host of minor parties blow off steam. (It also works well, of course, in the case where there are only three candidates.)

But Egypt wasn’t like this: four candidates placed close together in the first round, with the two who were eliminated having 17.5% and 20.7% against Morsi’s 24.8% and Shafiq’s 23.7%.

More significantly, the two eliminated candidates were regarded as the most liberal in the field: if one of them had made it to the runoff (or if there was a preferential voting system), he probably would have attracted most of the other’s supporters and therefore been able to beat either Morsi or Shafiq.

Instead, Morsi faced off against a representative of the old regime, whom he duly beat – no doubt with the support of many liberals who nonetheless found Morsi’s Islamist values deeply alien.

This sort of stuff is dry, but it’s essential – yet of course none of the media want to talk about it. ABC news tonight was telling us that Morsi was elected with “just over half the vote” – which of course as a description of the second round is quite accurate, but fails completely to tell anyone what actually mattered about the election.

Morsi’s problem is that he was the first choice of only a quarter of the electorate – an electorate that was (and obviously still is) polarised and with little experience of democracy. That put him in an unenviable position from the very start, and he hasn’t handled it well. His Muslim Brotherhood, despite winning an election, never represented the majority of the population, but he seems not to have been as conscious of that as he should have.

Having said that, it’s also true that the opposition has shown little willingness to play by the rules of democracy. Morsi has made offers for dialogue that have been repeatedly turned down, and opposition politicians have been too ready to boycott elections when they thought they were at a disadvantage. (A set of constitutional problems not of Mosri’s making haven’t helped.)

So Mosri, who is hardly a committed democrat, now appears as the champion of constitutional democracy. As the BBC quotes him, “If we changed someone in office who [was elected] according to constitutional legitimacy – well, there will be people opposing the new president too, and a week or a month later they will ask him to step down.”

The crowds in the streets of Egypt’s cities seem unimpressed by that argument. But how far they’re willing to take their discontent, and what the government and the army might do about it, remain to be seen.