In the early days of the AppStore you could drop your price to $0, then switch to paid, and retain your ranking. Apple quickly quashed this, so perhaps these apps are looking to benefit from word of mouth, or to gain a number of reviews. As I mentioned earlier you need 5-6 reviews before you even get a rating and having more certainly doesn't hurt.
On a similar note, I believe Apple's charts may now be weighted to take price into account, at least in some form. I occasionally play around with app prices to see how sales/revenue/in-app purchases change, and last week I upped the price of one app from $1.99 to $2.99. Sales dropped from ~400 to ~250, but my chart placing was virtually unchanged (I think I dropped once place). I don't believe the ranking is based on sales over a period of days, though it may be a factor, since my ranking does increase/decrease with normal sales fluctuations.
On a similar note, I believe Apple's charts may now be weighted to take price into account, at least in some form. I occasionally play around with app prices to see how sales/revenue/in-app purchases change, and last week I upped the price of one app from $1.99 to $2.99. Sales dropped from ~400 to ~250, but my chart placing was virtually unchanged (I think I dropped once place). I don't believe the ranking is based on sales over a period of days, though it may be a factor, since my ranking does increase/decrease with normal sales fluctuations.