What's With The Seeding System?

All along the years I thought, and I know thats the way it used to be earlier, that there was a method of sorting out the seeds in tournaments. Atleast from the quarter finals onwards it was pretty much determined. In the on going US Open Tennis I was just noticing the way the seeds were placed. In the quarter finals, it always used to be 1 vs 8, 2vs 7, 3 vs 6 and 4 vs 5. So according to this 5th seed Roddick should have been up against Davydenko and not the favourite and world number one, Federer. Davydenko is scheduled to play Federer in the Semis (1 vs 4) but the quarters finals I feel, should also have the same system. I understand that they probably cant adjust all 32 seeds like that but atleast for the top 8 seeds, it should be arranged.