This suggests that frictional unemployment may have declined and the labour market has become more efficient at matching workers and jobs. It isn't hard to imagine that the shift to online job advertising has had something to do with this. So there's one reason why the NAIRU might be lower nowadays.
Graph 6  The textbooks also talk about ‘structural unemployment’ as being part of the NAIRU. These are the people who for a variety of reasons cannot find any kind of job at prevailing wage rates. This might be because they lack the appropriate skills, or they have caring responsibilities, disabilities, or other circumstances that make it hard to find a suitable position.