This assumption turns out to be pretty accurate. It is assumed that the number of an item that is required on average is proportional to the number currently on the auction house. AuctionDB attempts to factor this into its market value. This is an inherent weakness with most market value estimates. If you bought the 15 cheapest, you'd pay 16.3 gold per item. If you bought the 5 cheapest auctions in this example, you'd pay 12.2 gold per item. It is easy to see that the value of the item depends on how many is typically bought at a time. AuctionDB calculates the market value in multiple steps which attempt to correct for outliers, give a moving value over time, and give a much more accurate market value in general than a simple average.
Performing a simple average of this data set would give you a market value of 25.79 which is obviously too high. Let's use the following data set as an example (assume these are gold values and each number represents the buyout of a single item): AuctionDB uses an algorithm with a bunch of steps to make it as accurate as possible while using as little memory / disk space to store the data as possible. This page attempts to explain exactly how AuctionDB calculates the market value of items.