A 6.9% rental vacancy rate implies an average vacancy of ~3.5 weeks per year. Given the high turnover of many rentals, that seems pretty low to me. Turnaround time just to do a make ready for a new tenant tends to be a week at minimum, sometimes longer for proper overhauls (replacing carpet, fixes damage, etc). Sure, not all properties turn over every year, but there's also quite a few properties that have longer vacancies to counteract that.