They need to find what vacancies are out there, work out if the job (and the firm) is a good match for their skills and interests, and then go through a selection process. How long these activities take will help determine how many people are between jobs even in a tight market.
The overall rate of job turnover will also matter, because that helps determine the flow of people into job search. All of those things can change, for example  because technological change makes job search or selection processes more efficient, or because transaction costs make it more expensive to move for a job.