Overture's distribution partners only show the top 2,3, and sometimes 4 listings. Especially for Yahoo, the top listings are highly coveted.
Google slaps a handful of ads down the side of the page. Their partner AskJeeves displays them all as the first part of their results.
Thus, Google is less competitive because the real estate isn't as much of an issue. This works out well because Google can algo the relevance better.