in most tennis tournaments, after the top 2 seeds, the remaining seeds are assigned randomly, many times actually drawn out of a bag or hat.
1 seed always on top, 2 seed always on bottom
3 and 4 seeds get randomly assigned to a half
5-8 seeds are randomly assigned to a quarter
9-16 seeds are randomly assigned to an eighth
17-32 seeds are randomly assigned to a sixteenth
this is just how it's always been done. since the computer-generated rankings fluctuate so gradually, but also so frequently, there is really no reason to have such rigid seeding guidelines.