The degree of coarseness or what is also known as hair shaft diameter plays a big role in this. The higher level of coarseness, the less grafts someone will need, all other factors being equal. Hair caliber is the single most important factor in attaining the illusion of coverge, even over density.
In addition, some patients just don't have the accomodating scalp laxity, even on a virgin scalp, although rare to see. This also would limit the maximum number of grafts harvested in strip.