Of course it is not 'guaranteed'.
However, the only data we have, suggests that these girls are the most popular girls. We have voting for a DOA5 tapestry back before LR came out where Marie Rose was in 2nd place just a point behind first, and then the voting for the members of DOAX3 where she came in first and Honoka in second.
There is 0 data as far as I am aware that suggests these girls are not the most popular, so I don't really understand what the point is in arguing it. I am not saying that that is what you specifically are arguing, but the other poster didn't seem to want to believe it.
Of course ideally you would want one vote for one person for a popularity contest, however, doing one vote for one person isn't really realistically possible over the internet, and is easily abused.
I imagine that in this case, the larger sample is 'people who are interested in DOAX', and that treating sales of themes of girls in bikinis as a vote, is a relatively good way to have a smaller sample size that represents that.
Maybe my idea of a fan is different. Taking your band/music artist example, I'm not sure I would consider someone who just listens to some songs on youtube, but never buys the groups CDs, to be a 'fan' compared to someone who does.