I think the GP actually meant a third of eligible voters, not of those who actually vote. Turnout is often not that great in countries that don't legally require voting, especially when voters are dissatisfied with the choices they have. In the 2024 US presidential election, for example, ~89 million eligible voters did not vote, which puts Trump's win at 32% of eligible voters, just under a third.
Whether or not this is a useful/interesting metric is debatable, I suppose.
Swing voters / swing states / whatever the particular country-specific analog is -- this is a symptom of poor voting systems; unfortunately the people in power like how this works, and so it won't change.
It's also, at least in the US, a collective action problem: a state that tries to be more "fair" (doesn't gerrymander, perhaps awards presidential electoral votes in proportion to the popular vote percentages, etc.) will end up giving seats/votes to the "other side" more often than polarized states that stick with FPTP.