It depends on the nature of user. If user is a newbie then he can click more on ads because of confusion (i.e. in main download button and ad download button).
The second factor is "where ad is placed?" it means if one places an ad near to any important link then the chance of click on ad is high.
Last but not least while ad networks prohibits to do this but still people do it, they change the appearance of ads in such a way...