I don't think anyone knows the answer to this but we all have our own opinions which is great. I can say that I have been #1 on Yahoo out of 20 million for 15 months and everytime I check I am the only one that validated my code. The fact that I use CSS and my content is 60% compared to the others at 20% or less has an effect on this.
Also I dont think it makes a difference how much competition there is, it could be 300,000 in this case or 3 million or 30 million to have 6 places in the top 10 and those are the only sites that use valid code has to say something. I took the 6 places in the last 4 months when I added Portland, Oregon to my web site. Before that none of the top 10 validated.
We have all agreed to disagree which is what makes a forum so interesting and informative.