I always thought better ads would show more often. if one ad has better ad copy and landing page then wouldnt it make sense for Google to show that ad more? Also other factors like the ads appearing on different datacenters might effect the over all viewing of the ad right?

@Jaan Kanellis: If you have set your campaign to "optimize," then you are correct - the ad with the higher CTR will show more often. But you can change this to "rotate" and ads will show more evenly. This is better for testing purposes, especially if you are optimizing for conversion rather than CTR.

I know that if you have the content network enabled, it can really throw off your ad serving, even if you have the campaign set to "rotate." We always shut off content if we are testing - or better yet, create separate content campaigns.

I called them on this once (literally and figuratively) after also setting Campaign Settings to "Rotate: Show ads more evenly" and seeing one of the ads continue to be served 95% of the time.

Funny thing is The Googler put me on hold while just conducting a search for the answer in AdWords help (I had just done the same before picking up the phone).

Here was their response verbatim:
"Higher-quality ads are more likely to receive better placement on a page, therefore gaining more impressions than other ads in the ad group. This may result in uneven ad served percentages between these ads. "

Thanks for 'learning me' how to read!