Another aspect of spare capacity that isn't always fully appreciated is the scope for extra workers to come directly from outside the labour force, or from incumbent workers working more hours, rather than from the ranks of the currently unemployed. The more that extra labour demand is met from new entrants or longer hours, the slower the unemployment rate declines.
So we can think of broader notions of spare capacity than the NAIRU. These would explain why the economy might grow at seemingly above-potential rates for longer without hitting capacity constraints. This has been a factor here in Australia, where labour force participation has increased sharply over the past year, especially for women (Graph 2).