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?