I sort of cynically assume that the layoff takes the subsequent attrition into account. Like I imagine they actually wanted to cut 10%, so they do 8% and let attrition take care of that remaining 2%.
If you are current at headcount X and want to be at headcount Y by the end of the year, you don't let go X-Y people: you need to take into account the expected voluntary attrition for the year as well as any critical hiring you may need to do.