Why do you think this happens?

The conversion rate increases the more time you spend on search. For example the first 1k you spend in search might yield a 1% conversion rate, yet the next 2k might yield a 3 or 4%. Without any refinement of the campaign. Is that because not all searches are unique, and time spent on a keyword term does in fact matter?

Or, do you think it comes from latent conversions. Ya know, people shopping around, and then search a 2nd , and 3rd time, and then convert.

Any opinions, or market research?

:)

I think the more you spend, the better ad spot Google gives ya :) Perhaps there are some general trends such as AdWords on page 1 of search results generally tend to convert better than AdWords on page 2, completely irrelevant of CTR. I also think the shopping around theory comes into play. I know that evaluating my traffic trends has shown that people browse around forums a good amount of time before registering or posting. I, myself, often find myself a long time lurker, first time poster, on a lot of forums.

Yeah, and I forgot to take into consideration the Google "Quality Score" which factors in many variables. That also must contribute.
Here is Google's definition of the Quality Score:

Quality Score

This is the basis for measuring the quality of your keyword and determining your minimum bid. Quality Score is determined by your keyword's clickthrough rate (CTR), relevance of your ad text, historical keyword performance, the quality of your ad's landing page, and other relevancy factors.

Shim - you beat me to the punch. Over time if your ad is performing it will advance higher in the rank... and thus will achieve even more clicks. Then further increase and so on.

Now is the time factor true with Yahoo? if it is -then latency may be something to consider. If not - it is simply Google's gift to you.

Generally am skeptical on latency. The other factor is that your brand is gaining more awareness or other channels are kicking in driving people to search...

There are many factors but I'd answer the Yahoo question - that will give tell more of the tale.

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