In recent months, we have noticed sporadic and apparently random changes in the way the App Store displays the most popular free and paid applications, Apple modifying the algorithms based on which it makes these listings. I say that the changes are unusual because I have seen new/old applications on "big" places without them being promoted intensively, the iDevice.ro application appearing out of the blue in the top, although there was no increase in the number of downloads or uses during that period.
Apple has been harshly criticized for the way those charts are manipulated and it seems that the company's engineers are trying to solve the problems, but I think that it will be a good few months until Apple finds a really good solution to the problem. I expect that during this time applications will appear "out of nowhere" on the first places, I expect the manipulation to continue and I recommend you not to rely on reviews or charts when buying an application.
